Composite member

ABSTRACT

A composite member suitable for a heat radiation member of a semiconductor element and a method of manufacturing the same are provided. This composite member is a composite of magnesium or a magnesium alloy and SiC, and it has porosity lower than 3%. This composite member can be manufactured by forming an oxide film on a surface of raw material SiC, arranging coated SiC having the oxide film formed in a cast, and infiltrating this coated SiC aggregate with a molten metal (magnesium or the magnesium alloy). The porosity of the composite member can be lowered by improving wettability between SiC and the molten metal by forming the oxide film. According to this manufacturing method, a composite member having excellent thermal characteristics such as a coefficient of thermal expansion not lower than 4 ppm/K and not higher than 10 ppm/K and thermal conductivity not lower than 180 W/m·K can be manufactured.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/122,365, filed Jun. 27, 2011, which is the U.S. National Phase under35 U.S.C. §371 of International Application No. PCT/JP2009/005132, filedon Oct. 2, 2009, which in turn claims the benefit of JapaneseApplication No. 2008-259000, filed on Oct. 3, 2008, Japanese ApplicationNo. 2008-261522, filed on Oct. 8, 2008, Japanese Application No.2009-230338, Oct. 2, 2009, Japanese Application No. 2009-230339, filedon Oct. 2, 2009 and Japanese Application No. 2009-230340, filed on Oct.2, 2009, the disclosures of which Applications are incorporated byreference herein.

TECHNICAL FIELD

The present invention relates to a composite member made of a compositeof magnesium (what is called pure magnesium) or a magnesium alloy and anon-metal inorganic material such as SiC and to a method ofmanufacturing a composite member. In particular, the present inventionrelates to a composite member suitable for a heat radiation member of asemiconductor element.

BACKGROUND ART

Not only a constituent material composed only of a metal material suchas copper but also a composite material made of a metal and a non-metalinorganic material (representatively, ceramics), such as Al—SiC, havebeen made use of as a constituent material for a heat radiation member(a heat spreader) of a semiconductor element. Mainly aiming to achievelighter weight of a heat radiation member, a composite materialincluding magnesium (Mg) which is lighter in weight than aluminum (Al)or an alloy thereof as a base material has recently been studied (seePatent Document 1).

Patent Document 1 discloses successful manufacturing with goodoperability, a composite material excellent in thermal characteristics(high in thermal conductivity and small in difference in coefficient ofthermal expansion from a semiconductor element or peripherals thereof)by arranging an aggregate of raw material SiC in a cast (a mold) andinfiltrating the aggregate with a molten metal (magnesium) in thatstate.

PRIOR ART DOCUMENTS Patent Documents Patent Document 1: Japanese PatentLaying-Open No. 2006-299304 SUMMARY OF THE INVENTION Problems to beSolved by the Invention

A composite material made use of as a constituent material for the heatradiation member above is desired to achieve the following:

(1) Further improvement in thermal characteristics;

(2) Improvement in joint characteristics with a cooling apparatus towhich a semiconductor element or a heat radiation member is attached;and

(3) Larger size.

(1) As to Improvement in Thermal Characteristics

A heat radiation member of a semiconductor element is desired to haveexcellent thermal conductivity and excellent adaptability in coefficientof thermal expansion with the semiconductor element or peripheralsthereof (approximately from 4 ppm/K (4×10⁻⁶/K) to 8 ppm/K (8×10⁻⁶/K)).Namely, development of a composite material having thermal conductivityequal to or higher than that of the composite material disclosed inPatent Document 1, that has a value further closer to a coefficient ofthermal expansion of the semiconductor element or peripherals thereof,such as a value close to approximately 4.5 ppm/K of an insulatingsubstrate or 3.5 ppm/K of a silicon package, has been desired.

Here, pores may be present in the conventional composite material (seeparagraph 0003 of Patent Document 1). By decreasing these pores, thatis, by lowering porosity of the composite material, improvement inthermal characteristics is expected. Patent Document 1, however, has notsufficiently studied how much porosity should be lowered. In addition, atechnique for lowering porosity has not sufficiently been studiedeither.

Porosity can be lowered by raising an infiltration temperature (atemperature of a molten metal in infiltration). In order to considerablylower porosity (for example, to less than 3%), the infiltrationtemperature should be set to a temperature exceeding 1000° C., and sucha high temperature is highly likely to cause such a defect as ashrinkage cavity or a gas hole at the time of solidification, whichleads to deterioration in a casting surface or lowering in thermalcharacteristics. Therefore, it is difficult to lower porosity and toimprove thermal characteristics with a method of raising an infiltrationtemperature. In addition, the method of raising an infiltrationtemperature requires a large-scale apparatus for heating in infiltrationand a life of a cast (a mold) is also short, which leads to increase incost.

Alternatively, porosity can be lowered by mixing an infiltration agentsuch as SiO₂ in a raw material. In order to considerably lower porosity(for example, to less than 3%), however, an infiltration agent should beincreased. If SiO₂ is employed as the infiltration agent, a reactionproduct such as an Mg—Si compound or an Mg oxide results. Such a productis low in thermal conductivity and hence use of a large amount of SiO₂as the infiltration agent leads to lowering in thermal characteristicsattributed to the product above. Therefore, it is again difficult tolower porosity and to improve thermal characteristics with this method.

As another technique for improving thermal characteristics, for example,in further lowering a coefficient of thermal expansion of the compositematerial above, it is effective to increase a content of a non-metalinorganic material in the composite material. For example, however, witha manufacturing method of filling a cast with non-metal inorganicmaterial powders followed by tapping (providing vibration) or pressingafter filling in the cast and infiltrating a gap between particles ofthe non-metal inorganic material with a molten metal, an amount ofpowders with which the cast can be filled is limited (see paragraph 0015of the specification of Patent Document 1). Therefore, the content ofthe non-metal inorganic material in the composite material obtained withthe manufacturing method above is at most 70 volume %.

(2) As to Joint Characteristics with a Semiconductor Element or the Like

If a semiconductor element should sufficiently be cooled, a heatradiation member and a semiconductor element or a heat radiation memberand a cooling apparatus are joined to each other with solder in somecases. The composite material, however, has poor solderability. Inaddition, Mg or an alloy thereof is poorer than Al in corrosionresistance. Then, one surface (a mount surface on which a semiconductorelement is to be mounted) or opposing surfaces (a mount surface and acooled surface opposed to this mount surface and in contact with acooling apparatus) of a substrate made of the composite material is(are) plated with nickel (Ni) or the like, in order to enhancesolderability or corrosion resistance.

It is difficult, however, to plate the conventional composite materialwith Ni or the like.

The surface of the composite material has many irregularities becauseSiC is present in a scattered manner, and it is difficult to uniformlyapply plating onto the surface. In order to smoothen the irregularities,the composite material may be subjected to surface polishing or rolling,however, such treatment is also difficult because SiC has high hardness.

In addition, taking into account productivity, electroplating ispreferred as plating above, however, SiC present at the surface of thecomposite material has high electrical insulating property andconduction cannot be achieved. Therefore, electroplating cannotsubstantially be performed. Though electroless plating can be performed,it is difficult to uniformly apply plating onto the surface because ofsurface irregularities as described above and in addition cost isincreased.

(3) As to a Larger Size

With more sophisticated functions and higher-density mounting in variouselectronic devices such as a personal computer and a portable electronicdevice, the number of semiconductor elements or peripherals thereofmounted on one circuit board tends to increase. A large-sized heatradiation member has accordingly been desired. Though Patent Document 1discloses a plate member having a size of 50 mm×100 mm as a compositematerial of Mg or an Mg alloy and SiC or the like, a further larger sizehas been desired. A method of manufacturing a larger composite material,however, has not conventionally been studied sufficiently. As a resultof studies conducted by the present inventors, it was found that aportion where defects are concentrated is formed in a large-sizedcomposite material, which may lead to lowering in grade or lowering inthermal characteristics, as will be described later.

From the foregoing, one object of the present invention is to provide acomposite member of Mg or an Mg alloy and SiC, that is low in porosity.

Another object of the present invention is to provide a composite memberof Mg or an Mg alloy and SiC, having thermal characteristics suitablefor a heat radiation member of a semiconductor element.

In addition, another object of the present invention is to provide acomposite member containing an Mg—SiC composite material as a mainconstituent material, that is readily subjected to electroplating.

Moreover, another object of the present invention is to provide acomposite member manufacturing method capable of manufacturing ahigh-grade composite member of Mg or an Mg alloy and SiC, in spite ofits large size.

Additionally, another object of the present invention is to provide acomposite member manufacturing method suitable for manufacturing thecomposite member above.

Means for Solving the Problems

The present inventors found that a magnesium-based composite materialhaving good thermal characteristics is obtained by achieving porositylower than 3%. In addition, the present inventors found that amagnesium-based composite material low in porosity is obtained by usingraw material SiC after it is subjected to a specific treatment or bysubjecting a composite solidified substance to a specific treatment,without excessively raising an infiltration temperature and withoutusing SiO₂ as an infiltrating agent. The present invention is based onthese findings.

A composite member according to the present invention in a first form isa composite member made of a composite of magnesium or a magnesium alloyand SiC, and it is characterized by having porosity lower than 3%.

The composite member according to the present invention in the firstform above can be manufactured with a composite member manufacturingmethod (1-1) or (1-2) according to the present invention shown below.

[Manufacturing Method (1-1)]

The manufacturing method (1-1) is a method of manufacturing a compositemember made of a composite of magnesium or a magnesium alloy and SiC,and it includes an oxidation treatment step and a composite step below.

Oxidation treatment step: Heating raw material SiC and forming an oxidefilm on a surface thereof.

In this oxidation treatment step, the oxide film is formed such that aheating temperature is set to 700° C. or higher and a mass ratio withrespect to the raw material SiC above satisfies a ratio not lower than0.4% and not higher than 1.5%.

Composite step: Arranging coated SiC having the oxide film above formedin a cast and infiltrating an aggregate of the coated SiC in the castwith magnesium or a magnesium alloy molten at a temperature not lowerthan 675° C. and not higher than 1000° C.

[Manufacturing Method (1-2)]

The manufacturing method (1-2) is a method of manufacturing a compositemember made of a composite of magnesium or a magnesium alloy and SiC,and it includes a preparation step and a pressurization step below.

Preparation step: Preparing a composite solidified substance of SiC andmagnesium or a magnesium alloy.

Pressurization step: Pressurizing the composite solidified substanceabove so as to collapse pores present in this composite solidifiedsubstance.

In this pressurization step, the composite solidified substance above ispressurized at a temperature not lower than room temperature and lowerthan a melting point of magnesium or the magnesium alloy above (aliquidus temperature), with a pressure not lower than 1 ton/cm².

According to the manufacturing method (1-1) in the present invention,the oxide film mainly composed of SiO₂ having high magnesium wettabilityis present on the surface of raw material SiC (powders or molded body).Therefore, as the aggregate of the coated SiC is infiltrated with themolten metal (a melt of magnesium or the magnesium alloy), sufficientcontact between the oxide film and the molten metal is achieved and thusthe molten metal readily surrounds SiC. Thus, according to themanufacturing method (1-1) in the present invention, pores produced dueto insufficient surrounding by the molten metal can effectively bedecreased and a composite member having porosity lower than 3% isobtained. In addition, according to the manufacturing method (1-1) inthe present invention, by employing coated SiC, SiO₂ having highmagnesium wettability can be present on the surface of raw material SiCin a concentrated manner and thus SiO₂ does not exist excessively in theraw material and a ratio of presence of a reaction product in theobtained composite member can also be lowered. Therefore, according tothe manufacturing method (1-1) in the present invention, lowering inthermal characteristics due to presence of the reaction product issuppressed and a composite member having excellent thermalcharacteristics is obtained.

According to the manufacturing method (1-2) in the present invention,pores can positively be eliminated by pressing the composite solidifiedsubstance under a specific condition and hence a composite member low inporosity is obtained. In particular, according to the manufacturingmethod (1-2) in the present invention, regardless of presence/absence ofan infiltration agent (SiO₂), a composite member having porosity nothigher than 5% and in particular lower than 3% is obtained. In a casewhere an infiltration agent (SiO₂) is not employed in the manufacturingmethod (1-2) in the present invention, an amount of generated reactionproduct which becomes a factor of lowering in thermal characteristicscan effectively be reduced.

In addition, according to the manufacturing method (1-1), (1-2) in thepresent invention, since it is not necessary to excessively heat themolten metal, such defects as a shrinkage cavity and a gas hole are lesslikely in the composite member and deterioration in thermalcharacteristics attributed to these defects is less likely. Therefore,according to the manufacturing method (1-1), (1-2) in the presentinvention, a composite member having excellent thermal characteristicsis obtained.

The composite member according to the present invention in the firstform above has very low porosity and excellent thermal characteristics.In particular, being manufactured with the manufacturing method (1-1) or(1-2) according to the present invention described above, the compositemember according to the present invention in the first form is less inlowering in thermal characteristics attributed to a reaction product orpresence of a shrinkage cavity or a gas hole, and excellent in thermalcharacteristics.

Further, the present inventors found that thermal characteristics of acomposite material can be improved by satisfying at least one offabricating a composite material having a network portion, for example,by making use of an SiC aggregate having a network portion bonding SiCto each other, and increasing a content of SiC in the composite materialby making use of the SiC aggregate formed with a specific method. Thepresent invention is based on the finding above.

The composite member according to the present invention in a second formis a composite member made of a composite of magnesium or a magnesiumalloy and SiC, and it has a coefficient of thermal expansion (acoefficient of linear thermal expansion) not lower than 4 ppm/K and nothigher than 8 ppm/K. In particular, the composite member according tothe present invention has a network portion containing 50 volume % ormore SiC above and bonding SiC above to each other. Other forms of thecomposite member having a coefficient of thermal expansion not lowerthan 4 ppm/K and not higher than 8 ppm/K include a form having nonetwork portion and containing more than 70 volume % SiC above.

The composite member according to the present invention in which the SiCaggregate in the composite member has the network portion above or thecomposite member containing a very large amount of SiC such as more than70 volume % (hereinafter referred to as a high SiC composite member) canbe lower in coefficient of thermal expansion than a conventionalcomposite material, and it can substantially be equal in coefficient ofthermal expansion to a semiconductor element or peripherals thereof(approximately from 4 ppm/K to 8 ppm/K). In particular, as thesecomposite members are mainly composed of a material such as SiC higherin thermal conductivity than Mg, they are high in thermal conductivityand in addition a continuous pathway (path) for heat conduction isformed therein by such a metal as magnesium. Thus, they are excellent inheat radiation characteristics. The composite member according to thepresent invention, which is a composite of a non-metal inorganicmaterial such as SiC and a metal material, or a high SiC compositemember described above has a low coefficient of thermal expansionadapted to a semiconductor element or the like, and also has excellentheat radiation characteristics and can suitably be made use of for aheat radiation member of a semiconductor element.

Meanwhile, there is normally a pore (air) in a gap between non-metalinorganic materials in a member composed only of a non-metal inorganicmaterial and there are fewer continuous paths for thermal conductionabove. In contrast, in the composite member according to the presentinvention or in the high SiC composite member described above, since thegap above is filled with such a metal as magnesium higher in thermalconductivity than air, the entire composite member serves as acontinuous path for heat conduction and it is excellent in thermalconductivity. In addition, there is a dense sintered body havingrelative density of approximately 99% as a member composed of anon-metal inorganic material. Even though such a dense sintered body iscompared with the composite member according to the present invention orwith the high SiC composite member described above, however, a compositemember containing such a metal as magnesium is advantageous in that (1)there are many continuous paths for heat conduction and thermalconductivity is high, (2) a coefficient of thermal expansion can beadjusted by adjusting a content or composition of magnesium or the like,(3) plating with Ni or the like is readily applied or chipping isreadily prevented because of ease in obtaining a smooth surface, asdescribed above, and in addition (4) manufacturing cost is low.

Moreover, the composite member according to the present invention in thesecond form or the high SiC composite member described above isexcellent in adaptability in coefficient of thermal expansion to asemiconductor element or peripherals thereof. Therefore, since thermalstress caused in a portion of joint to a semiconductor element or thelike is small and prescribed joint strength can be maintained,reliability of a semiconductor device including a heat radiation membercan be enhanced. Further, since the composite member according to thepresent invention in the second form or the high SiC composite memberdescribed above is excellent in thermal conductivity as described above,it enhances reliability as a heat radiation member and in addition theheat radiation member can be reduced in size, thus contributing also toreduction in size of a semiconductor device.

The high SiC composite member above can be manufactured, for example,with the following composite member manufacturing method. This compositemember manufacturing method is a method for manufacturing a compositemember made of a composite of magnesium or a magnesium alloy and SiC,and it includes a molding step and a composite step below. Thismanufacturing method will be referred to as an SiC high-density fillingmethod hereinafter.

Molding step: Step of forming an SiC aggregate by using any one of slipcasting, pressure forming and a doctor blade method.

Composite step: Step of forming the composite member containing morethan 70 volume % SiC above by infiltrating the SiC aggregate aboveaccommodated in a cast with molten magnesium or magnesium alloy in anatmosphere at a pressure not higher than an atmospheric pressure.

The composite member according to the present invention in the secondform above can be manufactured, for example, with the followingcomposite member manufacturing method according to the presentinvention. A composite member manufacturing method according to thepresent invention is a method for manufacturing a composite member madeof a composite of magnesium or a magnesium alloy and SiC, and itincludes a molding step, a sintering step and a composite step below.This manufacturing method will be referred to as a sintering methodhereinafter.

Molding step: Step of forming an SiC powder molded body.

Sintering step: Step of sintering the powder molded body to form an SiCaggregate having a network portion bonding SiC to each other.

Composite step: Step of forming the composite member having the networkportion above and containing 50 volume % or more SiC above byinfiltrating the SiC aggregate accommodated in a cast with moltenmagnesium or magnesium alloy in an atmosphere at a pressure not higherthan an atmospheric pressure.

Alternatively, as a method of manufacturing the composite memberaccording to the present invention in the second form above, a compositemember manufacturing method of manufacturing a composite member made ofa composite of magnesium or a magnesium alloy and SiC, including amolding step, a bonding step and a composite step below is exemplified.This manufacturing method will be referred to as a sol-gel methodhereinafter.

Molding step: Step of forming an SiC powder molded body.

Bonding step: Step of impregnating the powder molded body above with asolution of a precursor of a non-metal inorganic material followed byheating, generating a non-metal inorganic material based on theprecursor above, and forming an SiC aggregate in which SiC above isbonded to each other by a network portion composed of the generatednon-metal inorganic material.

Composite step: Step of forming the composite member having the networkportion above and containing 50 volume % or more SiC above byinfiltrating the SiC aggregate above accommodated in a cast with moltenmagnesium or magnesium alloy in an atmosphere at a pressure not higherthan an atmospheric pressure.

Alternatively, as another manufacturing method of manufacturing thecomposite member according to the present invention in the second formabove, a composite member manufacturing method of manufacturing acomposite member made of a composite of magnesium or a magnesium alloyand SiC, including a molding step and a composite step below isexemplified. This manufacturing method will be referred to as a reactionbonding method hereinafter.

Molding step: Step of forming a powder molded body by using a powdermixture of SiC powders and powders for reaction containing at least oneof boron and oxygen.

Composite step: Infiltrating the powder molded body above accommodatedin a cast with molten magnesium or magnesium alloy in an atmosphere at apressure not higher than an atmospheric pressure. In addition,generating a product composed of a new non-metal inorganic materialthrough reaction between the powders for reaction above and a moltenmagnesium component so as to bond SiC above to each other. Then, a stepof forming a composite member having a network portion composed of thisnew product and containing 50 volume % or more SiC above.

Alternatively, as another manufacturing method of manufacturing thecomposite member according to the present invention in the second formabove, a composite member manufacturing method of manufacturing acomposite member made of a composite of magnesium or a magnesium alloyand SiC, including a molding step, a sintering step and a composite stepbelow is exemplified. This manufacturing method will be referred to as areaction sintering method hereinafter.

Molding step: Step of forming a powder molded body by using a powdermixture of SiC powders and precursor powders for generating an oxide ora nitride through reaction to nitrogen or oxygen.

Sintering step: Sintering the powder molded body above in a nitrogenatmosphere or in an oxygen atmosphere and generating the nitride or theoxide above through reaction between the precursor powders above andnitrogen or oxygen. Then, a step of forming an SiC aggregate in whichSiC above is bonded to each other by a network portion composed of thisproduct.

Composite step: Step of forming the composite member having the networkportion above and containing 50 volume % or more SiC above byinfiltrating the SiC aggregate above accommodated in a cast with moltenmagnesium or magnesium alloy in an atmosphere at a pressure not higherthan an atmospheric pressure.

By forming an SiC aggregate with a specific method or by forming an SiCaggregate of which SiC filling rate was increased, such as a sinteredbody, as described above, a high-density SiC aggregate can readily bemanufactured. Then, by making a composite of such a high-density SiCaggregate and molten magnesium or magnesium alloy (hereinafter referredto as molten Mg), a composite member containing more than 70 volume %SiC can readily be manufactured. In addition, by forming an SiCaggregate having a network portion bonding SiC to each other or byforming the network portion above during infiltration, the compositemember according to the present invention in the second form in whichthe network portion above is present can readily be manufactured. Theobtained composite member has a coefficient of thermal expansionsatisfying 4 ppm/K to 8 ppm/K.

In particular, as compared with the SiC high-density filling methodabove, the sintering method or the sol-gel method above is advantageousin that (1) a composite member low in coefficient of thermal expansionand high in thermal conductivity is obtained because the compositemember having the network portion above is obtained, (2) a compositemember containing more SiC can readily be manufactured, and (3) the SiCaggregate having the network portion above has high strength andarrangement thereof in a cast or the like is easy. Moreover, by formingthe powder molded body in the sintering method or the sol-gel methodabove with a method predefined in the SiC high-density filling methodabove, a dense SiC aggregate is obtained. By making use of such an SiCaggregate, a composite member further higher in SiC content andexcellent in thermal characteristics is obtained.

In particular, in the sintering method above, sintering is carried outsuch that SiC is directly bonded to each other, that is, the networkportion is composed of SiC. Then, the non-metal inorganic materialpresent in the composite member is substantially SiC alone. Since SiC isparticularly high in thermal conductivity, this composite member hashigh thermal conductivity.

In particular, in the sol-gel method above, though a solution of aprecursor is necessary, a network portion can be formed by heating to atemperature lower than a sintering temperature in the sintering methodabove or a network portion can be formed at room temperature without anyheating depending on a type of a solution. Alternatively, by making useof a precursor with which SiC particularly high in thermal conductivityamong non-metal inorganic materials is generated and by forming anetwork portion with SiC, a composite member high in thermalconductivity is obtained as described above.

Meanwhile, with the reaction bonding method above, a composite memberhaving the network portion above can readily be manufactured withoutincluding a sintering step or a separate heating step, and in addition,a network portion can be generated simultaneously with making acomposite of the SiC aggregate and molten Mg. Therefore, the reactionbonding method is excellent also in manufacturability of a compositemember. With the reaction sintering method above, a network portion canbe generated even if a sintering temperature is lower than in a casewhere sintering is performed such that SiC is directly bonded to eachother. With the reaction bonding method or the reaction sinteringmethod, a network portion may be composed of a non-metal inorganicmaterial lower in thermal conductivity than SiC. Therefore, consideringimprovement in thermal conductivity, the sintering method or the sol-gelmethod above is preferred.

Alternatively, the composite member according to the present inventionin the second form can be formed also by preparing a commerciallyavailable SiC sintered body and infiltrating this SiC sintered body withmolten Mg. An SiC sintered body having an SiC content in the compositemember not lower than 50 volume % and having a network portion that canbe present in the composite member and open pores for infiltration withmolten Mg should be selected as appropriate as the SiC sintered bodyabove.

In addition, the present invention proposes, as a composite member towhich electroplating is readily applied, a form including a metalcoating layer on at least one surface of a substrate composed of acomposite material. A composite member according to the presentinvention in a third form includes a substrate composed of a compositematerial made of a composite of magnesium or a magnesium alloy and SiCand a metal coating layer covering at least one surface of thissubstrate. The substrate above contains 50 volume % or more SiC.

According to the composite member in the present invention in the thirdform, one surface of the substrate composed of the composite material iscovered with the metal coating layer having conductivity, and henceconduction is achieved. Therefore, electroplating can be applied. Inaddition, at least one surface of the substrate including the metalcoating layer will have less irregularities attributed to presence ofSiC, and thus uniform plating is more likely to be achieved. Moreover,since uniform plating can be applied, this composite member can achieveenhanced solderability and also enhanced corrosion resistance.Therefore, the composite member according to the present invention inthe third form can suitably be made use of as a heat radiation member.

The composite member according to the present invention in the thirdform above can be manufactured, for example, with the followingmanufacturing method. This manufacturing method is a method ofmanufacturing a composite member including a substrate composed of acomposite material made of a composite of magnesium or magnesium alloyabove and SiC by infiltrating an SiC aggregate accommodated in a castwith molten magnesium or magnesium alloy. In particular, in thismanufacturing method, an unfilled region not filled with SiC is providedbetween the cast above and the SiC aggregate above and a metal is causedto be present in this unfilled region, so that this metal forms themetal coating layer covering at least one surface of the substrateabove. This manufacturing method is hereinafter referred to as acomposite integration method.

Alternatively, the following method is exemplified as anothermanufacturing method of manufacturing the composite member according tothe present invention in the third form above. The composite membermanufacturing method according to the present invention is a method ofmanufacturing a composite member including a substrate composed of acomposite material made of a composite of magnesium or magnesium alloyabove and SiC by infiltrating an SiC aggregate accommodated in a castwith molten magnesium or magnesium alloy. In particular, thismanufacturing method according to the present invention includes a metalcoating layer formation step of stacking a metal plate on the substrateabove and pressurizing this stack with a pressure not lower than 0.5ton/cm² while this stack is heated to a temperature not lower than 300°C. This manufacturing method is hereinafter referred to as a hotpressing method.

The composite member according to the present invention in the thirdform including the metal coating layer can readily be manufactured withthe manufacturing method above (the composite integration method, thehot pressing method).

Additionally, in order to study problems arising in manufacturing alarge-sized heat radiation member, the present inventors fabricated aninfiltrated plate by infiltrating a plate-shaped composite member havingsuch a size as exceeding 50 mm×100 mm, specifically, a sufficient sizefrom which a circular region having a diameter exceeding 50 mm can betaken, and made of a composite of Mg or an Mg alloy and a non-metalinorganic material, with the use of an infiltration method as describedin Patent Document 1 (a method of infiltrating a non-metal inorganicmaterial, with which a cast is filled, with molten Mg or Mg alloy(hereinafter referred to as molten Mg) and thereafter solidifying moltenMg), and gradually cooled the infiltrated plate in an atmospherefurnace. Then, the obtained plate-shaped composite member was cut in adirection of thickness thereof and the cross-section was observed. Then,defects were present in a concentrated manner in a central portion ofthe composite member and substantially no defect was observed in aperipheral portion. More specifically, a large void (pore) caused by ashrinkage cavity (an internal shrinkage cavity) was observed in theinside of the central portion above. In addition, external shrinkagecavities or irregularities were also observed on a surface of thecentral portion of the composite member above. The reason why defectswere locally caused as such is estimated as follows.

By gradually cooling the infiltrated plate above, solidification ofmolten Mg proceeds in such a manner as converging from a peripherythereof toward the central portion and from the surface to the inside.Then, a hot spot (a region of an unsolidified metal surrounded by asolidified metal) may be produced in the inside of the central portionabove. Here, Mg or the Mg alloy decreases in volume by approximately 4%when it solidifies. Therefore, solidification proceeds in such a mannerthat unsolidified Mg or Mg alloy compensates for decrease in volumecaused by solidification. Then, when the metal not in a solid solutionstate in the hot spot in the central portion above is solidified, astate is such that Mg or the Mg alloy is short due to compensation aboveand thus compensation as described above is not carried out.Consequently, it is considered that defects are less likely in theperipheral portion of the plate-shaped composite member but a large voidis likely to be formed in the central portion. In addition, even thoughmolten Mg at a surface portion in the central portion above has alreadysolidified, pulling toward the inside occurs due to decrease in volumeat the time of solidification of the unsolidified portion inside.Therefore, it is considered that an external shrinkage cavity is causedat the surface of the central portion above or irregularities conformingto an outer geometry of a non-metal inorganic material used as a rawmaterial are produced due to the shrinkage at the surface. Then, iflarge defects as described above are present in a concentrated manner inthe inside of the composite member, such mechanical characteristics asstrength or thermal characteristics such as thermal conductivity arelowered. Here, in a case where a plate-shaped composite member is madeuse of as a heat radiation member of a semiconductor element, normally,a semiconductor element or the like is mounted on the central portion ofthe composite member. Therefore, a composite member in which defects areconcentrated in a portion where a semiconductor element is to be mountedis not suitable for a heat radiation member. Meanwhile, presence of sucha surface defect as an external shrinkage cavity as described above inan outer portion of the composite member leads to deterioration insurface property or lowering in dimension accuracy. In particular, sincethe composite material of such a non-metal inorganic material as SiC anda metal contains SiC cooled more readily than the molten metal, aninternal defect or a surface defect above is more likely than in a casewhere only the molten metal is used to form a cast member composed onlyof metal.

The present inventors found that, in order to decrease local defectsabove, cooling in one direction from one peripheral portion of theinfiltrated plate toward the other opposing peripheral portion insteadof convergent cooling of the infiltrated plate from the periphery towardthe central portion is effective. The present invention is based on thefinding above.

A composite member manufacturing method according to the presentinvention is a method of manufacturing a composite member made of acomposite of magnesium or a magnesium alloy and SiC, and it includes aninfiltration step and a cooling step below. This manufacturing method ishereinafter referred to as a one-direction cooling method.

Infiltration step: Step of forming an infiltrated plate by infiltratingan SiC aggregate accommodated in a cast with molten magnesium ormagnesium alloy.

Cooling step: Step of cooling the infiltrated plate above in onedirection from a side of the infiltrated plate above opposite to a sideof supply of the molten magnesium or magnesium alloy above so as tosolidify the molten magnesium or magnesium alloy above.

Representatively, in the infiltration step above, the molten magnesiumor magnesium alloy above is supplied to the aggregate above in adirection of gravity by its own weight, that is, from an upper side in avertical direction toward a lower side in the vertical direction. Then,in the cooling step above, cooling of the infiltrated plate above in onedirection from the lower side of the infiltrated plate in the verticaldirection toward the upper side thereof in the vertical direction, tothereby solidify the molten magnesium or magnesium alloy above, isexemplified.

The composite member manufacturing method according to the presentinvention above can naturally be made use of for manufacturing asmall-sized composite member from which a circular region having adiameter not greater than 50 mm can be taken, however, in particular, itcan suitably be made use of for a case of manufacturing a compositemember having a sufficient size from which a circular region having adiameter exceeding 50 mm can be taken. In this case, the infiltratedplate above should only have a sufficient size from which a circularregion having a diameter exceeding 50 mm can be taken.

In addition, according to the composite member manufacturing method inthe present invention above, for example, a composite member accordingto the present invention in a fourth form below is obtained. Thiscomposite member is a plate member made of a composite of magnesium or amagnesium alloy and SiC, and it has a sufficient size from which acircular region having a diameter exceeding 50 mm can be taken. Then,with a straight line passing through the center of gravity when thecomposite member is two-dimensionally viewed being defined as a sectionline, a cross-section in a direction of thickness of the compositemember is taken. A range in this cross-section extending over up to 10%of a length of the section line along a longitudinal direction of thesection is defined as a central region (this central region willhereinafter be referred to as a central region (defect)), with thecenter of gravity serving as a center. Any small region of 1 mm×1 mm istaken from this central region (defect), and when a ratio of an area ofa defect portion with respect to an area of the small region is definedas an area ratio, the composite member has the area ratio above nothigher than 10%.

As the direction of supply of molten Mg is set, for example, to thedirection of gravity as described above in the manufacturing methodaccording to the present invention above (the one-direction coolingmethod), molten Mg not in a solid solution state in the infiltratedplate above unexceptionally moves from the upper side in the verticaldirection (representatively, an open side of a cast) toward the lowerside in the vertical direction (representatively, a bottom surface sideof the cast) by its own weight. When the infiltrated plate above iscooled in one direction from the lower side in the vertical directiontoward the upper side in the vertical direction in this state,unsolidified Mg on the upper side is successively supplied to the lowerside as molten Mg on the lower side solidifies. Namely, solidificationproceeds toward the upper side in the vertical direction while volumedecrease at the time of solidification of molten Mg on the lower side inthe vertical direction above is successively compensated for byunsolidified molten Mg present on the upper side in the verticaldirection above. Therefore, with this manufacturing method, as inconvergent cooling from the periphery of the infiltrated plate towardthe central portion as described above, concentration of large defectsin the central portion (a portion including the central region (defect)above) of the composite member is unlikely. Thus, according to thecomposite member in the present invention in the fourth form obtained bysetting the cooling direction to one specific direction, large defectsare not present in a concentrated manner and the composite member is ofhigh grade. In addition, the composite member according to the presentinvention in the fourth form above manufactured with the manufacturingmethod according to the present invention (the one-direction coolingmethod) has a small defect portion such as a void attributed to aninternal shrinkage cavity in the central portion including the centralregion (defect) above, in spite of being a large-sized plate memberhaving a sufficient size from which a circular region having a diameterexceeding 50 mm can be taken. Thus, the composite member according tothe present invention in the fourth form in which no large defect islocally present is excellent in such mechanical characteristics asstrength or such thermal characteristics as thermal conductivity, andadditionally it can sufficiently have an area for mounting asemiconductor element or peripherals thereof. Therefore, this compositemember can suitably be made use of for a heat radiation member in thesemiconductor element above.

The present invention will be described hereinafter in further detail.

[Composite Member]

As a form of the composite member according to the present invention, aform including a substrate alone composed of a composite material madeof a composite of magnesium or a magnesium alloy and a non-metalinorganic material (mainly SiC) and a form including the substrate aboveand a metal coating layer covering at least one surface of thissubstrate are exemplified. Initially, the substrate above will bedescribed.

(Substrate)

<Metal Component>

What is called pure magnesium composed of 99.8 mass % or more Mg and animpurity or a magnesium alloy composed of an added element and remainderMg and an impurity is adopted as a metal component in the substrateabove. A case where pure magnesium is adopted as the metal componentabove is more advantageous than a case where an alloy is adopted in that(1) thermal conductivity of the composite member is enhanced and (2) thecomposite material having a uniform texture is readily obtained becausesuch a disadvantage as non-uniform precipitation of a crystallizedproduct at the time of solidification is less likely. In a case wherethe magnesium alloy is adopted as the metal component above, a liquidustemperature becomes lower and thus a temperature in melting can belowered, and in addition corrosion resistance and mechanicalcharacteristics (such as strength) of the composite member can beenhanced. At least one of Li, Ag, Ni, Ca, Al, Zn, Mn, Si, Cu, and Zr isexemplified as the added element. Increase in content of these elementsleads to lowering in thermal conductivity, and therefore these elementsare preferably not more than 20 mass % in total (assuming the metalcomponent as 100 mass %; to be understood similarly hereinafter as tothe content of the added element). In particular, preferably, Al is notmore than 3 mass %, Zn is not more than 5 mass %, and each of otherelements is not more than 10 mass %. Addition of Li is effective forlighter weight of the composite member and improvement in workability. Aknown magnesium alloy such as AZ-type, AS-type, AM-type, ZK-type,ZC-type, or LA-type may be employed. A metal raw material is prepared soas to attain a desired composition.

<Non-Metal Inorganic Material>

<<Composition>>

A non-metal inorganic material lower in coefficient of thermal expansionthan Mg, excellent in thermal conductivity, and less likely to react toMg is exemplified as a non-metal inorganic material in the substrateabove. Such ceramics as SiC is representative of such non-metalinorganic materials. Other than the above, at least one of Si₃N₄, Si,MgO, Mg₃N₂, Mg₂Si, MgB₂, MgCl₂, Al₂O₃, AlN, CaO, CaCl₂, ZrO₂, diamond,graphite, h-BN, c-BN, B₄C, Y₂O₃, and NaCl can be exemplified. Inparticular, SiC is adopted in the present invention, because SiC (1) hasa coefficient of thermal expansion approximately from 3 ppm/K to 4 ppm/Kwhich is close to a coefficient of thermal expansion of a semiconductorelement or peripherals thereof, (2) has particularly high thermalconductivity among non-metal inorganic materials (single crystal:approximately from 390 W/m·K to 490 W/m·K), (3) is commerciallyavailable as powders or sintered bodies in various shapes and sizes, and(4) has high mechanical strength. The present invention permitsnon-metal inorganic materials listed above other than SiC to becontained. Namely, the composite member according to the presentinvention may contain a plurality of types of non-metal inorganicmaterials above. The non-metal inorganic material other than SiC ispresent, for example, as a network portion.

<<Shape>>

SiC in the composite member according to the present invention ispresent representatively in a form randomly dispersed in magnesium or amagnesium alloy (hereinafter referred to as a dispersed form) or in aform bonded by the network portion (hereinafter referred to as a bondedform). In particular in the bonded form having the network portion, sucha form that SiC as a whole is continuous as being bonded by the networkportion and a gap between SiC and SiC is filled with magnesium or amagnesium alloy, that is, a porous body which will have open pores ifmagnesium or the magnesium alloy should be removed, is preferred. Inparticular, this porous body is preferably has fewer closed pores.Specifically, preferably, a ratio of closed pores to the total volume ofthe non-metal inorganic material in the composite member is not higherthan 10 volume % and preferably not higher than 3 volume %.

Regarding the non-metal inorganic material such as SiC in the compositemember, the non-metal inorganic material used as the raw material ispresent substantially as it is. For example, in a case where powders ofthe non-metal inorganic material are made use of as the raw material,they are in the dispersed form above. In a case where a molded body ofthe non-metal inorganic material above, in particular, a porous moldedbody having a network portion bonding particles to each other throughheating or the like of the powder molded body, is made use of as the rawmaterial, the non-metal inorganic material is in the bonded form aboveor in the dispersed form above due to collapse or the like of thenetwork portion in the molded body above at the time of infiltration. Byadjusting a sintering condition or the like as appropriate, a state ofpresence of the non-metal inorganic material in the substrate can bevaried.

For example, when a porous body having few closed pores as describedabove is made use of as the raw material SiC aggregate, a path forimpregnation with molten Mg can sufficiently be ensured and the obtainedcomposite member itself has fewer pores because the open pores above arefilled with molten Mg. Fewer pores lead to higher thermal conductivityof this composite member. A shape of a mold to be filled with rawmaterial powders or a shape of the porous body above as a whole to bemade use of as the raw material is preferably selected as appropriatesuch that the composite member (substrate) has a prescribed shape.Presence of a network portion or a ratio of closed pores in thecomposite member can be checked or measured, for example, by observing across-section of the composite member with an optical microscope or ascanning electron microscope (SEM).

<<Content>>

If the content of SiC in the substrate above is not less than 20 volume% with this substrate being assumed as 100 volume %, a substrate high inthermal conductivity κ and low in coefficient of thermal expansion (acoefficient of linear thermal expansion) a can be obtained. As thecontent of SiC in the substrate is greater, thermal conductivity κ tendsto become higher and coefficient of thermal expansion α tends to belower, and hence adaptability in coefficient of thermal expansion to asemiconductor element (approximately from 4 ppm/K to 8 ppm/K (forexample, Si: 4.2 ppm/K, GaAs: 6.5 ppm/K)) or peripherals thereof (aninsulating substrate: approximately 4.5 ppm/K, a silicon package:approximately 3 ppm/K, an alumina package: 6.5 ppm/K, a metal package(stainless steel (around 20 ppm/K), steel (11 to 12 ppm/K))) is likelyto be obtained. Therefore, the SiC content is preferably not lower than50 volume % in any form of the bonded form and the dispersed form above.More preferably, the SiC content is not lower than 55 volume %,particularly preferably not lower than 70 volume %, further preferablynot lower than 75 volume %, preferably not lower than 80 volume % amongothers, and still further preferably not lower than 85 volume %. Theupper limit of the SiC content is not particularly provided. If the SiCcontent exceeds 90 volume %, large pressurization force is required informing a high-density SiC aggregate to be used as the raw material orformation of closed pores is more likely in a subsequent step such assintering, which may lead to an SiC aggregate in which closed poresoccupy more than 10 volume %. Therefore, taking into account industrialproductivity and infiltration performance with molten Mg, the SiCcontent approximately from 80 volume % to 90 volume % is considered aspractical. In the bonded form having a network portion, the SiC contentis set to 50 volume % or higher. In the high SiC composite member in thedispersed form not having a network portion, the SiC content is set tomore than 70 volume %.

The composite member (substrate) containing more than 50 volume % SiChas high thermal conductivity κ satisfying 180 W/m·K. Although dependingon the SiC content, the form of a network portion, composition of themetal component, or the like, the composite member having thermalconductivity κ not lower than 200 W/m·K, particularly not lower than 250W/m·K and further not lower than 300 W/m·K can be obtained.

In addition, if the SiC content in the composite member (substrate) isnot lower than 50 volume % and not higher than 80 volume %, it is likelythat a coefficient of thermal expansion from 4×10⁻⁶ to 10×10⁻⁶/K (4 to10 ppm/K) is satisfied. If the content exceeds 80 volume %, it is likelythat the coefficient of thermal expansion is lower than 4 ppm/K and sucha composite member is likely to well adapt to a semiconductor element orthe like further lower in coefficient of thermal expansion.

Since the content of a non-metal inorganic material in the compositemember is substantially equal to an amount of a raw material, an amountof a raw material is preferably selected as appropriate such that thecomposite member (substrate) has desired thermal characteristics.

<<Purity>>

If high-purity SiC is used as raw material SiC and purity of SiC presentin the composite member is high such as 95% or higher and particularly99% or higher, characteristics of SiC can sufficiently be made use of.In addition, use of highly crystalline SiC is likely to bring abouthigher thermal conductivity of the composite member.

<<Material for Network Portion>>

In a case where the non-metal inorganic material in the composite memberhas a network portion, a metal material such as Mo in addition to thenon-metal inorganic material such as SiC is exemplified as a constituentmaterial for the network portion. In a case where the network portion isalso composed of SiC, this composite member is substantially composed ofSiC and magnesium (or the magnesium alloy), and it has excellent heatradiation characteristics as described above. A silicon nitride (Si₃N₄),a magnesium compound (such as MgB₂, MgO or Mg₃N₂), other nitrides (suchas BN and MN), and oxides (such as CaO) are exemplified as othernon-metal inorganic materials forming a network portion. In particular,Si₃N₄ has a low coefficient of thermal expansion and a composite memberlow in coefficient of thermal expansion can be obtained.

<<Thickness of Network Portion>>

The present inventors examined a composite member including a networkportion. Then, even though the SiC content in the composite member wasequivalent, thermal characteristics were different. As a result ofinvestigation of the cause, the form of the network portion in thecomposite member was different. Specifically, with any line segmenthaving a prescribed length being taken in a cross-section of thecomposite member, a length of a portion where a contour line of an SiCaggregate constituted of SiC and the network portion crosses the linesegment above, that is, a length between adjacent intersections, thatare intersections between the contour line above and the line segmentabove, was different. A composite member longer in length between theintersections above, that is, a composite member having a thick networkportion, tends to be excellent in thermal characteristics and inparticular low in coefficient of thermal expansion. A composite membershorter in length between the intersections above, that is, a compositemember having a thin network portion, tends to be excellent inmechanical characteristics and in particular high in tensile strength orbending strength. Then, a thick network portion is defined as aconstruction excellent in thermal characteristics. Specifically, as thenetwork portion becomes thicker, the number of intersections in the linesegment above decreases. Therefore, taking any line segment having alength of 1 mm with respect to an actual dimension of the compositemember in the cross-section of the composite member, a composite memberin which the number of intersections between the contour line of the SiCaggregate constituted of SiC above and the network portion above and theline segment above satisfies a value not larger than 50 is proposed.Since it is expected that a smaller number of intersections above leadsto further superior thermal characteristics, the lower limit of thenumber of intersections above is not particularly set. Meanwhile, whenthe number of intersections above exceeds 50 and particularly it is 100or more, further superior mechanical characteristics are expected. Inorder to change a thickness of the network portion above, for example,adjustment of a manufacturing condition or the like which will bedescribed later is exemplified.

<Thickness of Substrate>

A thickness of the substrate above can be selected as appropriate. In acase where this substrate is made use of as a heat radiation member of asemiconductor element, the thickness is preferably not larger than 10 mmand particularly preferably not larger than 5 mm. When the substrate hasa thickness approximately not larger than 10 mm, a substrate notincluding a metal coating layer which will be described later, inparticular a substrate containing a large amount of SiC such as morethan 70 volume % or a substrate having a network portion, can have acoefficient of thermal expansion approximately from 4 ppm/K to 6 ppm/K.

<Texture>

<<Porosity>>

The composite member according to the present invention in the firstform obtained with the manufacturing method (1-1) according to thepresent invention in which wettability between a molten metal and SiChas been enhanced or with the manufacturing method (1-2) according tothe present invention in which pores are collapsed and eliminated haslow porosity. Specific porosity is lower than 3%, preferably lower than1%, and further preferably lower than 0.5%. Desirably, no pore ispresent, and ultimately porosity of 0% is desired. A method of measuringporosity will be described later.

<<Oxygen Concentration>>

An oxide film was positively formed on raw material SiC and a thicknessof the oxide film effective for lowering porosity was studied. Then, thethickness was more than 60 nm and approximately 320 nm. In the compositemember according to the present invention in the first form manufacturedwith the use of such coated SiC, it is considered that Mg oxide (such asMgO) generated as a result of reaction between a molten metal and theoxide film is present in a large amount in the vicinity of SiC,specifically, in a region within 100 to 300 nm from the contour line ofSiC. Therefore, in magnesium or the magnesium alloy present around SiCin the composite member, with a region in a shape similar to SiCextending from the contour line of SiC outward by 150 nm or shorterbeing assumed as an SiC outer peripheral region and a region extendingfrom the contour line of SiC outward by 1 μm or more being assumed as amain region, a composite member in which oxygen concentration in the SiCouter peripheral region is higher than in the main region is exemplifiedas one form of the composite member according to the present invention.When coated SiC above is employed as the raw material, in across-section at any portion of the composite member, any SiC outerperipheral region is higher in oxygen concentration than the mainregion. Therefore, oxygen concentration can be examined for any SiCouter peripheral region at any portion of the composite member. In acase where the composite member is used for a heat radiation member,however, the central portion of the composite member is involved withheat radiation performance, and hence oxygen concentration in the SiCouter peripheral region present in this central portion is preferablyexamined. Namely, assuming a region inside relative to a regionextending over up to 1/10 of a minimal length of the composite memberfrom an outer periphery in the cross-section of the composite member asa central region (this central region hereinafter referred to as acentral region (oxygen)), oxygen concentration is examined for thiscentral region (oxygen).

For example, in a case where the composite member (substrate) is aplate-shaped member having a lateral width of 50 mm and a thickness ofapproximately 5 mm, the thickness of the composite member corresponds tothe minimal length above. Therefore, in this composite member, a regioninside relative to a region extending over 5 mm× 1/10=0.5 mm (500 μm)from the outer periphery of the composite member is assumed as thecentral region (oxygen). Specifically, a plane passing through a portioncorresponding to ½ of a thickness is defined as a central plane, acenter line in a direction of a lateral width of this central plane istaken, and a region extending from this center line in each direction ofthe lateral width by 24.5 mm and from the central plane in eachdirection of thickness by 2 mm is defined as the central region(oxygen). Alternatively, for example, in a case where the compositemember is a plate-shaped member having a thickness not greater than 1 mmas well, the thickness of the composite member corresponds to theminimal length. Therefore, a region inside relative to a regionextending over up to 1/10 of the thickness should only similarly bedefined as the central region (oxygen). In particular, when a thicknessis small, by defining a plane passing through a portion corresponding to½ of the thickness as the central plane, a region extending from thiscentral plane in each direction of thickness by approximately 100 μm,that is, a region having a thickness of approximately 200 μm, may bedefined as the central region (oxygen).

In a case of a composite member in the dispersed form in which SiC ispresent in a dispersed manner in the composite member, one or more pieceof SiC is selected in the central region (oxygen) above, the outerperipheral region and the main region are selected for selected SiC, andoxygen concentration is measured for comparison. More specifically, forexample, SiC is selected by using an EDX analyzer attached to a TEMapparatus, an outer peripheral region and a main region are furthertaken for each selected piece of SiC, and measurement points areselected in each of the outer peripheral region and the main region. Asa measurement point in the outer peripheral region, a point at adistance by 150 nm outward from the contour line of SiC (any point on anperiphery of the outer peripheral region) can be selected, and as ameasurement point in the main region, a portion on magnesium or amagnesium alloy at a distance by 3 μm outward from the contour line ofSiC, not including other SiC outer peripheral regions, can be selected.For one piece of SiC, five or more measurement points in the outerperipheral region and five or more measurement points in the main regionare selected for observation. Preferably, three or more (preferablyfive) pieces of SiC are selected, and one or more (preferably five ormore) measurement point(s) in the outer peripheral region and one ormore (preferably five or more) measurement point(s) in the main regionare selected for observation. In the case of the bonded form describedabove, the outer peripheral region and the main region above arepreferably selected for a contour line at any portion of SiC in thecomposite member.

Difference in oxygen concentration between the SiC outer peripheralregion and the main region can be checked, for example, by making use ofcharacteristic X-ray spectrochemical analysis, Auger electronspectrochemical analysis, or the like with the use of an EDX analyzerattached to an SEM apparatus or an EDX analyzer attached to a TEMapparatus.

<Internal Defect>

One of features of the composite member (substrate) according to thepresent invention in the fourth form is that defects are not present ina concentrated manner in the central region (defect) above in spite ofits large size as described above. Specifically, an area ratio of adefect portion in the small region above selected in the central region(defect) above is not higher than 10%. The defect portion above refersto a component in the small region above except for Mg or the Mg alloyand a non-metal inorganic material (mainly SiC), and a void isexemplified as a representative.

By adjusting as appropriate a cooling condition as will be describedlater, a composite member satisfying the area ratio above not higherthan 5% and particularly not higher than 2% can be obtained. Since adefect is desirably absent, the lower limit of the area ratio is notparticularly set. In a case where cooling in one specific direction asdescribed above is not performed, a composite member (substrate) havingthe area ratio above exceeding 10% is obtained. Namely, this substratehas many defects in the central region (defect) and these defects arevery large. More specifically, this substrate has a defect having a sizeto such an extent as visually recognizable, and such a large defect ispresent in the central region (defect) in an uneven manner. In contrast,the composite member (substrate) according to the present invention inthe fourth form has few defects in the central region (defect) and thisdefect has a size to such an extent that visual recognition is difficult(a largest length of the defect being not larger than approximately 0.1mm (100 μm)). In the composite member (substrate) according to thepresent invention in the fourth form, a large defect is not locallypresent, and even though a defect is present, it is very small.Therefore, this composite member (substrate) is superior in suchmechanical characteristics as strength and such thermal characteristicsas thermal conductivity to a case where large defects are present in aconcentrated manner in a part. In addition, in this composite member,defects are not present in a concentrated manner not only in the centralregion (defect) above but also over substantially the entire region ofthe cross-section above. Namely, this composite member has a uniformtexture, without uneven presence of defects in its entirety. If thecomposite member is in such a form as including a metal coating layer onthe surface of the substrate made of the composite material above, thecentral region (defect) above is extracted only from the substrateexcept for the metal coating layer.

<Surface State>

The composite member (substrate) according to the present invention inthe fourth form is free from uneven presence of internal defects asdescribed above and in addition, it has few surface defects such asexternal shrinkage cavities. Therefore, this composite member(substrate) has less irregularities in the surface and it can satisfysurface roughness Ra, for example, not greater than 2.5 μm. In addition,since the substrate itself has less surface roughness, even if the metalcoating layer is formed on the substrate by using molten Mgsimultaneously with making a composite, in particular even if a thinmetal coating layer having a thickness not greater than 1 mm is formed,the surface of the metal coating layer is also smooth and the metalcoating layer can satisfy surface roughness Ra not greater than 2.5 μm.The surface of the metal coating layer can be smoothened also by forminga thick metal coating layer and performing such machining as polishing.Meanwhile, since the metal coating layer may be thin as will bedescribed later, polishing above or the like will lead to lower yieldand increase in polishing cost.

In addition, the composite member (substrate) according to the presentinvention in the fourth form is less likely to deform because of fewersurface defects as described above and it is superior also in dimensionaccuracy. For example, this composite member (substrate) can satisfy adifference between a largest thickness and a smallest thickness of thecomposite member in the cross-section of the central region (defect)described above not larger than 0.2 mm and in particular not larger than0.05 mm (50 μm). In a case of including a metal coating layer on thesurface of the substrate as well, surface defects are few as describedabove and hence the difference above can satisfy 0.2 mm or smaller andin particular 0.05 mm (50 μm) or smaller.

<Size and Shape>

One of the features of the composite member (substrate) according to thepresent invention in the fourth form is its relatively large size. Asone form, a rectangular shape having a short side having a lengthexceeding 50 mm and a long side having a length exceeding 100 mm isexemplified. As the composite member (substrate) has a large size assuch, for example in making use of this composite member for aconstituent material for a heat radiation member of a semiconductordevice, a large number of semiconductor elements or peripherals thereofcan be mounted.

A size and a shape of a substrate can be selected as appropriate andthey are not particularly specified. For example, other than therectangular shape above, a square shape or a circular shape may beadopted. A cast is prepared such that a substrate has a desired size anda shape.

<Thermal Characteristics>

Although depending on a content of a non-metal inorganic material suchas SiC in the composite member or a form of presence of the non-metalinorganic material, composition of a metal component, porosity, a stateof presence of a defect, or the like, the composite member (substrate)according to the present invention has excellent thermal characteristicsas described above. Specifically, for example, the composite member canhave such high thermal conductivity as satisfying thermal conductivity κnot lower than 180 W/m·K, in particular not lower than 200 W/m·K,further not lower than 250 W/m·K, and not lower than 300 W/m·K amongothers. In addition, a substrate relatively low in coefficient ofthermal expansion as satisfying a coefficient of thermal expansion notlower than 3.5 ppm/K (3.5×10⁻⁶/K) and not higher than 20 ppm/K(20×10⁻⁶/K), in particular not lower than 4.0 ppm/K (4.0×10⁻⁶/K) and nothigher than 12 ppm/K (12×10⁻⁶/K), and further not lower than 4×10⁻⁶/K (4ppm/K) and not higher than 10×10⁻⁶/K (10 ppm/K), can be obtained. Such acomposite member according to the present invention is not onlyexcellent in thermal conductivity but also excellent in adaptability incoefficient of thermal expansion to a semiconductor element andperipherals thereof, and hence it can suitably be made use of for a heatradiation member of the semiconductor element.

(Metal Coating Layer)

<Composition, Texture>

By providing a metal coating layer on at least one surface of thesubstrate above and making use of this metal coating layer as a base forelectroplating as described above, plating with Ni or the like canreadily be applied to a substrate composed of the composite materialabove.

The metal coating layer mainly functions as the base for electroplatingwith Ni or the like, and hence a constituent metal of the metal coatinglayer should only be a metal having conductivity sufficient for beingable to achieve conduction required for electroplating, and it may havecomposition different from or the same as a metal component (Mg or theMg alloy) of the substrate made of the composite material above. Inparticular, when the composition is the same, in making a composite ofthe SiC aggregate and molten Mg in the manufacturing method describedabove, a part of molten Mg is made use of for forming the metal coatinglayer so that the metal coating layer is formed simultaneously withmaking a composite. Then, the composite member having the metal coatinglayer can be manufactured with good productivity. In this case, in theobtained composite member, the metal component in the substrate aboveand the metal forming the metal coating layer above have a continuoustexture (a cast texture).

In particular in a case where the metal coating layer is made of puremagnesium, Young's modulus of Mg is low and hence a large thickness ofthe metal coating layer is less likely to change a coefficient ofthermal expansion of the entire composite member. Therefore, a compositemember having a low coefficient of thermal expansion is readilyobtained.

When the metal component of the substrate above and the constituentmetal of the metal coating layer above are different from each other incomposition, the constituent metal for the metal coating layer may be,for example, an Mg alloy different in composition from the metalcomponent of the substrate above, or a metal other than Mg and the Mgalloy, such as aluminum (Al), copper (Cu), silver (Ag), gold (Au), zinc(Zn), nickel (Ni), and an alloy thereof. In particular, one metalselected from the group consisting of Mg, Al, Cu, and Ni of which purityis 99% or higher and an alloy mainly composed of Mg, Al, Cu, and Ni (analloy containing 50 mass % or more Mg, Al, Cu, and Ni; to be understoodsimilarly hereinafter) is preferred as the constituent metal for themetal coating layer. The metal above is close in solidus temperature toMg or the Mg alloy representing the metal component of the substrate,and hence it is excellent in adhesiveness to the metal component orexcellent in corrosion resistance. Thus, the metal above can effectivelysuppress corrosion of the substrate made of the composite material.

<Portion of Formation>

The metal coating layer above should only be present on a surface atleast requiring plating, among surfaces of the substrate above.Specifically, the metal coating layer is provided on at least one of amount surface on which a semiconductor element is to be mounted and acooling surface opposed to this mount surface and coming in contact witha cooling apparatus. A form in which the metal coating layer is providedon each of the mount surface and the cooling surface, that is, a formhaving two metal coating layers, may be adopted, and the metal coatinglayer may be provided on the entire surface including an end surface (asurface coupling the mount surface and the cooling surface above to eachother) of the substrate above. Not only electroplating can be applied tothe composite member including the metal coating layer, but alsocorrosion resistance thereof is enhanced, the surface thereof becomessmooth, and its appearance is excellent. Therefore, commercial values ofthe composite member can be enhanced.

<Thickness>

Too large a thickness of the metal coating layer will lead to loweringin thermal conductivity of the composite member and increase in a rateof thermal expansion. Therefore, each metal coating layer preferably hasa thickness not larger than 2.5 mm, further not larger than 1 mm, andamong others not larger than 0.5 mm. In a case where the metal coatinglayer is provided on each of the two opposing surfaces of the substrateas described above, if the sum of thicknesses of two metal coatinglayers is not larger than 2.5 mm, the entire composite member includingthe substrate and the metal coating layer in particular in the bondedform having a network portion is more likely to have a coefficient ofthermal expansion not higher than 8 ppm/K. In addition, if the sum ofthicknesses of the two metal coating layers above is not larger than 0.5mm, the entire composite member including the substrate and the metalcoating layer even in the dispersed form in particular is more likely tohave a coefficient of thermal expansion not higher than 8 ppm/K.Further, if each metal coating layer has a thickness not smaller than 1μm and among others not smaller than 0.05 mm (50 μm) and not larger than0.1 mm (100 μm), it is considered that the metal coating layersufficiently functions as the base for plating, and in addition themetal coating layer is less likely to be damaged during transportation,mounting or the like of the composite member. The metal coating layermay be formed to a large thickness and then it may be set to a desiredthickness through polishing or the like. In this case, a compositemember having excellent appearance is obtained.

A coefficient of thermal expansion of the composite member including themetal coating layer can easily be found by fabricating a test piece fromthe composite member and conducting measurement using a commerciallyavailable apparatus. Alternatively, a coefficient of thermal expansionof the composite member including the metal coating layer may becalculated based on mixture rules in consideration of rigidity or thelike of each material forming the composite member above.

Alternatively, in a case of making use of the manufacturing methodaccording to the present invention in which cooling in one specificdirection is performed as described above, even though the metal coatinglayer has a small thickness, the metal coating layer having excellentsurface property can be formed.

In a case where the substrate has a thickness not larger than 10 mm anda substrate alone has a low coefficient of thermal expansion asdescribed above, even though each metal coating layer above is formed toa large thickness around 1 mm (the total thickness around 2 mm), acoefficient of thermal expansion of the entire composite memberincluding the substrate and the metal coating layer above can be nothigher than 8 ppm/K.

[Application]

The composite member above can suitably be made use of for a heatradiation member. This heat radiation member is not only excellent inadaptability in coefficient of thermal expansion to a semiconductorelement and peripherals thereof but also high in thermal conductivity,and therefore it can suitably be made use of for a heat radiation memberof the semiconductor element.

In particular, the composite member obtained by using coated SiC(powders or molded body) is low in porosity and of high grade, and itscommercial value is high. A heat radiation member including the metalcoating layer is readily subjected to electroplating, and in addition ithas excellent surface property and it is high in commercial value. Eventhough the heat radiation member obtained with the one-direction coolingmethod described above is a large-sized composite member, it is freefrom locally present defects, it has few large internal defects and alsohas few surface defects, and it is also excellent in surface property.Therefore, this heat radiation member is expected to be of high gradeand to have high commercial values. In addition, since this heatradiation member has fewer defects as described above, it is excellentin such mechanical strength as strength and thermal characteristics aswell as large. Therefore, the heat radiation member can mount a largenumber of semiconductor elements or peripherals thereof.

A semiconductor device including the heat radiation member above and asemiconductor element mounted on this heat radiation member can suitablybe made use of as various parts of electronic devices.

[Manufacturing Method]

The composite member according to the present invention can bemanufactured by forming an SiC aggregate and making a composite of thisSiC aggregate and molten Mg (infiltration→solidification).

In manufacturing a composite member low in porosity in particular, SiChaving an oxide film formed thereon is used as a raw material.Alternatively, in manufacturing a composite member in a bonded formhaving a network portion, the network portion is formed with anappropriate method. In manufacturing a composite member including ametal coating layer, the metal coating layer is formed in the compositestep or the like described above, or the metal coating layer isseparately formed after the substrate made of the composite material ismanufactured. In manufacturing a large-sized composite member inparticular, molten Mg is solidified through cooling in one specificdirection.

<Raw Material>

SiC powders are mainly made use of as a raw material for the SiCaggregate above. In the case of a form in which a silicon nitride isgenerated in sintering in the sintering method described above, not onlythe SiC powders but also Si powders and powders of a compound containingSi are prepared so that this powder mixture can be made use of. In thereaction sintering method described above, not only the SiC powders butalso precursor powders described above (for example, SiCl₄, organic Sicompound) are prepared so that this powder mixture can be made use of.In the reaction bonding method described above, not only the SiC powdersbut also powders for reaction described above (for example, powders of asimple substance element such as boron, BN, TiB₂, boric acid (B₂O₃), andsodium tetraborate (Na₂B₄O₅(OH)₄.8H₂O)), powders of an oxide, a boride,and a boroxide are prepared so that this powder mixture can be made useof.

These powders may be particulate or fibrous. When a powder has anaverage particle size (in a fibrous case, an average short diameter) notsmaller than 1 μm and not larger than 3000 μm and in particular notsmaller than 10 μm and not larger than 200 μm, the SiC aggregate isreadily manufactured, which is preferred. In the case of the dispersedform, SiC is readily uniformly dispersed in Mg or the Mg alloy, which ispreferred.

In addition, use of a plurality of types of powders different in averageparticle size as combined is likely to further enhance a filling rate ofSiC or the like. In using SiC powders different in size, a large powderhaving an average particle size not smaller than 50 μm and not largerthan 1/10 of a shortest inner dimension of a cast and a small powderhaving an average particle size not smaller than 1/20 and not largerthan ½ of the average particle size of the large powder are more likelyto enhance the filling rate as described above. A size or a shape of SiCin the composite member can be checked, for example, by observing across-section of the composite member with an SEM, an opticalmicroscope, or the like. Use of SiC powders different in size can bechecked, for example, as follows. In a cross-sectional micrograph of thecomposite member (for example, ×100 magnification), regarding a field ofview including approximately 50 these pieces of SiC each having aparticle size not smaller than 3 μm, a particle size of each piece ofSiC having a particle size not smaller than 3 μm present in this fieldof view is measured, for example, with an intercept method. An averageand a standard deviation of the particle size of approximately 50 piecesof SiC are found. Then, when a value obtained by dividing the standarddeviation by the average is not smaller than 0.5, it can be determinedthat SiC different in size was used. Magnification in observation aboveis preferably adjusted as appropriate in accordance with a particle sizeof SiC such that approximately 50 SiC particles are viewed in the fieldof view.

<Molding Step: Formation of Powder Molded Body>

In the SiC high-density filling method described above, a powder moldedbody not having a network portion is formed with any one of slipcasting, pressure forming and a doctor blade method which will bedescribed later. The powder molded body obtained with these methods hasstrength sufficient for handling. In the sintering method, the sol-gelmethod, the reaction sintering method, and the reaction bonding methoddescribed above, the powder molded body is formed with tapping or thelike in addition to the method described above. In the case of thedispersed form, the SiC aggregate can readily be formed by tapping.

<<Slip Casting>>

The powder molded body can be formed with slip casting, by fabricatingslurry using raw material powders described above as well as water and adispersant and by molding this slurry followed by drying. A commonsurfactant can be made use of as the dispersant. Slip casting has suchadvantages that a molded body in a complicated shape can readily bemolded, a molded body high in filling rate (density) is obtained evenwith the use of fine powders, even a large-sized molded body can readilybe molded, and increase in cost for facilities is less.

<<Pressure Forming>>

Pressure forming includes dry pressing, wet pressing, single-shaftpressure forming, OP (cold isostatic press), and extrusion. In the caseof molding with dry pressing, the powder molded body can be formed bysubjecting raw material powders described above to pressure forming. Inthe case of molding with wet pressing, the powder molded body can beformed by subjecting a powder mixture in which raw material powders anda liquid such as water are mixed to pressure forming so as to press outa liquid. A pressure (molding pressure) at the time of pressure formingis preferably selected as appropriate. In any case of dry pressing andwet pressing, a binder made use of in powder molding can be made use ofas appropriate. Pressure forming has such advantages that a grain sizeof raw material powders is readily made uniform, the number of steps issmaller than in slip casting, and hence productivity is excellent.

<<Doctor Blade Method>>

The powder molded body can be formed by fabricating slurry with the useof raw material powders described above, a solvent, an antifoamingagent, a resin, and the like, pouring this slurry into an inlet port ofa doctor blade, forming a sheet-shaped body, and thereafter evaporatingthe solvent. The doctor blade method can suitably be made use of informing a plate-shaped molded body.

In the SiC high-density filling method described above, a powder moldedbody formed with slip casting or the like described above may be adoptedas the SiC aggregate or a sintered body obtained by further sinteringthis powder molded body may be adopted as the SiC aggregate, and then acomposite of the SiC aggregate and molten Mg may be made. For example,(1) a vacuum atmosphere, a heating temperature: 800 to less than 1300°C., and a holding time: approximately 2 hours, and (2) an atmosphere, aheating temperature: 800 to 1500° C., and a holding time: approximately2 hours are exemplified as sintering conditions here. By performingsintering under the sintering condition above, advantageously, (1)strength higher than the powder molded body above is achieved, chippingor the like is less likely at the time of accommodation in a cast, orthe like, and handling is easy, (2) a porous body can readily befabricated, and (3) an SiC filling rate can be improved by adjusting asintering temperature or a holding time so as to make the sintered bodydense, and thus a composite member of which SiC content is not lowerthan 70 volume % is readily obtained. In addition, through heatingduring sintering, a binder or the like used for fabricating the powdermolded body can be evaporated and removed. These advantages of thesintered body are also similar in a sintered body having a networkportion which will be described later. Under the sintering condition(1), (2) above, however, a composite member in the dispersed form nothaving a network portion tends to be obtained. Therefore, in obtaining acomposite member having a network portion and a low coefficient ofthermal expansion, sintering under a sintering condition which will bedescribed later is preferably performed.

<<Tapping>>

The powder molded body having a shape in conformity with a cast canreadily be formed by filling the cast (mold) with the raw materialpowders described above and applying constant vibration. It is difficultto form a high-density SiC aggregate with tapping, and tapping can bemade use of in a case where an SiC content in the composite member isapproximately 50 to 70 volume %. If the SiC content exceeds 70 volume %,such a method as slip casting described above is preferred.

The powder molded body containing such components as Si, a precursor andboric acid described above is formed with various methods above with theuse of the powder mixture described above, and in addition, it can befabricated by fabricating an SiC powder molded body consisting only ofSiC powders, thereafter impregnating the SiC powder molded body with aliquid mixture (for example, an aqueous solution) in which the Sipowders, the precursor powders, the powders for reaction (boric acid orsodium tetraborate), and the like above that are separately prepared aremixed in a solvent such as water, and thereafter drying the solventabove. By making use of the liquid mixture above, a desired substancesuch as Si can readily uniformly be dispersed in the powder molded body,and in addition, the powder molded body is formed without using powdersother than the SiC powders. Therefore, an amount of addition of the SiCpowders is not decreased and the SiC filling rate in the powder moldedbody is likely to increase.

<Sintering Step: Formation of Sintered Body>

In the sintering method described above, the powder molded body obtainedin the molding step above is sintered to fabricate an integrated SiCaggregate (sintered body) and to generate a network portion. Inparticular, with the sintering method described above, a network portionin the sintered body, that can be present also in the composite member,is positively formed as the network portion above.

A vacuum atmosphere, a heating temperature not lower than 1300° C. andnot higher than 2500° C., and a holding time for approximately 2 hoursare exemplified as the sintering condition above. By performingsintering under this condition, SiC can directly be bonded to eachother. Namely, a network portion can be formed of SiC. By directlybonding SiC to each other, strength of the sintered body is furtherincreased and in addition, a composite member having a low coefficientof thermal expansion and high thermal conductivity is likely to beobtained by using this sintered body. Moreover, a dense sintered body isobtained under the sintering condition as described above, and the SiCcontent (filling rate) in the composite member can be improved. Inparticular, when a sintering temperature is set to 2000° C. or higher, anetwork portion can have a greater thickness (satisfy the number ofintersections described above not greater than 50). When the sinteringtemperature is lower than 2000° C., a network portion tends to bethinner. The heating temperature or the holding time above is preferablyselected as appropriate in accordance with a form of the networkportion.

In the case of forming the powder molded body containing the Sicomponent described above, in the sintering step, the powder molded bodyabove is sintered in a nitrogen atmosphere to generate Si₃N₄ and thenetwork portion above can be formed with Si₃N₄ above. In the case ofthus forming the network portion, even if the heating temperature at thetime of sintering is as low as approximately 800 to 1800° C., SiC cansufficiently be bonded to each other and the network portion can have alarge thickness (satisfy the number of intersections described above notgreater than 50).

The powder molded body containing Si above can also be formed by usingan oxide containing Si, such as an additive composed of ceramics, forexample, SiO₂, H₂SiO₃, and Na₂SiO₃ and by reducing this oxide.Specifically, for example, the powder molded body is formed of a powdermixture of the SiC powders and powders of the oxide above and the powdermolded body above is reduced by using carbon powders or acarbon-containing gas, to thereby obtain the powder molded bodycontaining Si. Alternatively, the powder molded body containing Si isobtained by preparing the powder molded body formed of the SiC powdersand an aqueous solution of the oxide containing Si above, impregnatingthe powder molded body with the aqueous solution, and thereafterreducing the powder molded body as described above. In this case, as inthe case of making use of the liquid mixture described above, Si is morelikely to uniformly be dispersed and the SiC filling rate is readilyincreased. Commercially available carbon powders can suitably be madeuse of as the carbon powders above, and carbon monoxide (CO) orhydrocarbon such as methane (CH₄) high in reduction capability cansuitably be made use of as the carbon-containing gas above.

If a compound containing Si is made use of as a raw material, it isconsidered that an element except for Si is vaporized duringinfiltration or remains in a composite member as a compound with Mg orother compounds.

<<Bonding Step>>

In the sol-gel method described above, the powder molded body obtainedin the molding step above is impregnated with a solution of theprecursor described above followed by heating, so as to generate anon-metal inorganic material (for example, SiC, MgO, CaO) from theprecursor, and a network portion is formed of this non-metal inorganicmaterial and an integrated SiC aggregate is fabricated. For example,polycarbosilane, metal alkoxide and the like are exemplified as theprecursor above. A heating temperature is preferably selected asappropriate in accordance with the precursor above. The sol-gel methodis excellent in manufacturability of the SiC aggregate as compared withthe case of sintering described above, because a network portion can beformed at a lower heating temperature. In addition, in the case ofmaking use of polycarbosilane, SiC can newly be generated and hence SiCdensity is increased and a composite member high in SiC content isobtained.

<Formation of Oxide Film>

In addition, by making use of an SiC aggregate having an oxide filmformed on its surface as the SiC aggregate to be used together withmolten Mg, wettability between the SiC aggregate and molten Mg isenhanced, which is preferred. By preparing the SiC aggregate includingthe oxide film, even when the SiC content is high and a gap between SiCand SiC is very small, molten Mg readily penetrates owing tocapillarity. In obtaining a composite member having a network portion,preferably, an oxidation step of forming an oxide film is provided afterfabrication of the SiC aggregate such as the sintered body, so as toform a coated SiC molded body. In obtaining a composite member nothaving a network portion, representatively in using the SiC powdermolded body together with molten Mg, an oxide film is formed on a rawmaterial powder such as an SiC powder and then the powders including theoxide film are preferably used to form the SiC aggregate (coated SiCpowder molded body).

When a ratio of the oxide film to the SiC powders in the SiC molded bodysuch as the sintered body is low (an amount of oxide film is small), aneffect of improvement in wettability with a molten metal achieved by theoxide film is not sufficient. If the ratio is high (an amount of oxidefilm is large), an amount of Mg oxide which is a reaction productbecomes large, which leads to lowering in thermal characteristics. Theoxide film is formed such that a mass ratio of the oxide film to the SiCpowders or the SiC molded body is not lower than 0.4% and not higherthan 1.5%. Preferably, the oxide film is formed such that a mass ratiois not lower than 0.4% and not higher than 1%. More specifically, theoxide film is formed so as to have a thickness approximately from 50 nmto 300 nm. As the oxide film is larger in amount (thicker), infiltrationwith a molten metal at a lower temperature in a shorter period of timeis allowed. Therefore, for example, if such effects as simplification ofa heating apparatus for a cast during infiltration, reduction in aninfiltration time period (cycle time), and a longer life of a cast aredesired more than improvement in thermal characteristics, the SiCpowders or the SiC molded body including the oxide film of which massratio exceeds 1.5% can be made use of.

If a temperature for heating the SiC powders or the SiC molded body suchas the sintered body is lower than 700° C. in forming the oxide filmabove, the oxide film cannot sufficiently be formed on the surface ofSiC and it is not sufficiently wetted with the molten metal, which leadsto higher porosity of the composite member. Therefore, a heatingtemperature for forming the oxide film is set to 700° C. or higher.Oxidation reaction at the surface of SiC becomes active at 800° C., andas the heating temperature is higher, a rate of formation of the oxidefilm increases. Therefore, the heating temperature above is preferablynot lower than 800° C., in particular not lower than 850° C., andfurther not lower than 875° C. If the heating temperature is too high,however, the rate of formation of the oxide film becomes too fast andcontrol of an amount (thickness) or uniformity of the oxide film becomesdifficult. Increase in amount of oxide film or non-uniform thickness ofthe oxide film may lead to lowering in thermal conductivity due topresence of a large amount of reaction product or partially insufficientwetting with the molten metal. Therefore, the upper limit temperature ispreferably set to 1000° C. The amount of oxide film is also affected bya heating time period, and it tends to increase as the heating timeperiod is longer. A preferred heating time period is approximately 2hours. In addition, the amount of oxide film is also affected by a sizeof raw material SiC. When the SiC powders are used as the raw material,the amount of oxide film tends to increase as a particle is finer.Therefore, a heating temperature, a heating time period, a size of rawmaterial SiC, and the like are preferably adjusted as appropriate suchthat the amount of oxide film attains to a desired amount (mass ratio).In addition, when change in size of raw material SiC is desired whilethe total amount of oxide film is maintained at a prescribed amount, aheating temperature and a heating time period at the time of formationof the oxide film are preferably adjusted in correspondence with a sizeof SiC after change thereof. When the oxide film is formed, a portion inthe vicinity of SiC in the composite member described above (a regionwithin 100 to 300 nm from the contour line of the SiC aggregate) tendsto be high in oxygen concentration than a portion other than the portionin the vicinity.

<Composite Step: Formation of Substrate>

The composite member (substrate) is obtained by accommodating the SiCaggregate in a cast as described above, infiltrating the SiC aggregatewith molten Mg, and thereafter solidifying molten Mg. In the reactionbonding method described above, simultaneously with making a compositeof the SiC aggregate (powder molded body) above and molten Mg, boron oroxygen in the powder molded body above and molten Mg are caused to reactto each other, so as to generate a new product (boride or oxide) andthus a network portion can be formed of this product.

If the composite step of infiltrating the SiC aggregate above withmolten Mg is performed in an atmosphere at a pressure not higher than anatmospheric pressure (approximately 0.1 MPa (1 atm)), a gas in theatmosphere is less likely to be taken in and pores attributed to take-inof the gas are less likely to be caused in the composite member. SinceMg is high in vapor pressure, handling of molten Mg becomes difficult ifa high vacuum state is set. Therefore, in a case where a pressure of theatmosphere in the composite step above is set lower than the atmosphericpressure, a pressure not lower than 0.1×10⁻⁵ MPa is preferred.Alternatively, if the composite step above is performed in an inertatmosphere such as Ar, reaction between an Mg component and anatmospheric gas in particular can be prevented and deterioration inthermal characteristics attributed to presence of a reaction product canbe suppressed.

An infiltration temperature not lower than 650° C. is preferred. As theinfiltration temperature is higher, wettability between the SiCaggregate and molten Mg is enhanced, and hence an infiltrationtemperature not lower than 700° C., in particular not lower than 800°C., and further not lower than 850° C. is preferred. If the infiltrationtemperature exceeds 1000° C., however, such a defect as a shrinkagecavity or a gas hole may be caused or Mg may boil. Therefore, theinfiltration temperature not higher than 1000° C. is preferred. Inaddition, in order to suppress generation of an excessive oxide film orgeneration of a crystallized product, an infiltration temperature nothigher than 900° C. is preferred.

In using the SiC aggregate including the oxide film, the infiltrationtemperature is dependent on the amount (thickness) of oxide film. If theamount of oxide film is small in a range of the amount of oxide filmdescribed above, a high infiltration temperature is preferred. If theamount of oxide film in the range above is large, the infiltrationtemperature may be low. Specifically, when the amount of oxide film isnot lower than 0.4 mass % and not higher than 0.65 mass %, theinfiltration temperature not lower than 800° C. and not higher than1000° C. is preferred. When the amount of oxide film is higher than 0.65mass % and lower than 1.5%, the infiltration temperature may be notlower than 675° C. and not higher than 875° C. and naturally it may behigher than 875° C.

More specifically, for example, when pure magnesium is employed as amolten metal and an amount of oxide film is set to approximately 0.4mass % with respect to the total amount of raw material SiC, atemperature for infiltration with the molten metal exceeding 750° C.enhances wettability between the molten metal and the SiC aggregateincluding the oxide film and thus infiltration can be carried out. Asthe infiltration temperature is higher as described above, wettabilityis enhanced and porosity is lowered. Therefore, the infiltrationtemperature is preferably not lower than 800° C. and in particular notlower than 850° C. Alternatively, for example when pure magnesium isemployed as a molten metal and an amount of oxide film is set toapproximately 0.8 mass % with respect to the total amount of rawmaterial SiC, a temperature for infiltration with the molten metal setto 675° C. or higher enhances wettability between the molten metal andthe SiC aggregate including the oxide film and thus infiltration can becarried out. In this case as well, as in the case of the amount of oxidefilm above around 0.4 mass %, as the infiltration temperature is higher,wettability is enhanced and porosity is lowered. Therefore, theinfiltration temperature is preferably not lower than 700° C. and inparticular not lower than 725° C. If the amount of oxide film is greater(not higher than 1.5 mass %), it is considered that even a temperaturefor infiltration with the molten metal lower than 700° C. will allowgood infiltration. In making use of pure magnesium, however, in order toprevent occurrence of defects or boiling as described above, theinfiltration temperature is preferably not higher than 1000° C.

Meanwhile, if the magnesium alloy is employed as the molten metal, aliquidus temperature is lower than a melting point of pure magnesium andinfiltration at a lower temperature can be carried out. For example, ifAZ31 or AZ91 is employed as the molten metal, infiltration can becarried out even though the amount of oxide film above is small or eventhough the infiltration temperature is not higher than 800° C. If AZ91is employed as the molten metal, a temperature allowing infiltration isnot lower than 650° C. In making use of the magnesium alloy as well, asin the case of pure magnesium above, as the infiltration temperature ishigher, wettability is enhanced and porosity is lowered. Therefore, theinfiltration temperature is preferably not lower than 700° C. In thecase of the magnesium alloy, as compared with pure magnesium, a vaporpressure is lowered and a boiling point is raised. Therefore, theinfiltration temperature can slightly be higher and it can be set to1000° C. or higher. Even so, the infiltration temperature is preferablylower than 1100° C.

Solidification (cooling step) of the molten metal in the composite suchas an infiltrated plate is also preferably carried out in an inertatmosphere. A pressure of the atmosphere may be set to an atmosphericpressure, however, in order to suppress generation of defects at thetime of solidification, it may be set to a pressure not lower than anatmospheric pressure. The composite is preferably cooled rapidly inorder to suppress growth of a crystallized product. By employing a castformed of carbon, graphite, stainless steel, or the like excellent inthermal conductivity or by performing forced cooling such as watercooling, air cooling using a fan or the like in addition to leaving forcooling, a cooling rate can be increased. In particular, in forming alarge-sized composite member, cooling in one specific direction ispreferably performed. In forming a small-sized composite member,however, uniform cooling over the entirety may be carried out.

<Formation of Metal Coating Layer>

In forming the metal coating layer on the surface of the substratecomposed of the composite material above, a cast including a compositemember formation portion in a desired shape is made use of as the cast.A cast integrally including a composite member formation portion and aportion of placement of Mg or the Mg alloy representing a metalcomponent of the substrate (which may hereinafter be referred to as abase material metal) may be employed so that the base material metal ismolten by heating this cast. In the composite integration methoddescribed above, in particular, an unfilled region where filling withSiC is not performed is provided between the cast (composite memberformation portion) and the SiC aggregate, and a metal present in thisunfilled region forms a metal coating layer on at least one surface ofthe substrate.

A molten base material metal (molten Mg) and a separately prepared metalplate are exemplified as a metal caused to be present in the unfilledregion. In the case of the former base material metal, the obtainedcomposite member is made of the constituent metal of the metal coatinglayer and the metal component of the substrate having the samecomposition and the same texture. In the case of the latter metal plate,the obtained composite member is made of the constituent metal of themetal coating layer and the metal component of the substrate differentin composition or texture.

In the case of the former base material metal, the following methods (1)to (3) are exemplified as a method of forming a metal coating layer byhaving the molten base material metal (molten Mg) exist in the unfilledregion.

(1) SiC powders are used to form a molded body smaller in volume than acast (this molded body is employed as the SiC aggregate). This moldedbody is arranged in the cast and a gap is provided between the cast andthe molded body (this gap serves as the unfilled region). Then, inmaking a composite of the molded body and molten Mg, molten Mg is pouredalso into the gap above. Molten Mg or Mg alloy above poured into thisgap forms the metal coating layer.

(2) A spacer is arranged in the composite member formation portion inthe cast (this spacer serves as the unfilled region). After the SiCaggregate above is arranged in the composite member formation portion ofthe cast above where the spacer has been arranged, the spacer above isheated and vaporized for removal. As a result of removal of this spacer,a gap is produced between the cast (composite member formation portion)and the SiC aggregate. Then, in making a composite of the SiC aggregateand molten Mg, molten Mg is poured also into the gap above (a spacewhere the spacer has been present). Molten Mg or Mg alloy above pouredinto this gap forms the metal coating layer.

Alternatively, (2′) a form in which a spacer is not removed byvaporization, sublimation or the like can be adopted. In this case, amolded body such as a sintered body is preferably made use of as the SiCaggregate. Specifically, for example, the following is exemplified asthe manufacturing method according to the present invention ofmanufacturing a composite member having a metal coating layer.

SiC powders are used to form a molded body smaller in volume than thecast above, this molded body is adopted as the SiC aggregate above, themolded body above is arranged in the composite member formation portionin the cast above, and a spacer is arranged to maintain a gap betweenthe molded body and the cast above (this gap serves as the unfilledregion above). The metal coating layer above is formed of molten Mg orMg alloy above poured into the gap above.

In the form above, a composite member in which the spacer and the metalcoating layer are integrated is obtained. A spacer may remain as it isso that a composite member including the spacer may be made, or acomposite member from which a spacer portion is removed by suchmachining as cutting may be made. If a spacer is allowed to remain, aremoval step is not necessary, which leads to excellentmanufacturability. Since a spacer is present at the time of formation ofthe metal coating layer in the form above, the metal coating layer canbe formed in a stable manner while reliably maintaining the gap above.As a constituent material for the spacer, a material excellent in heatresistance, that is not removed by a melt of Mg or Mg alloy,representatively a material less likely to be vaporized, sublimated, ormolten, such as carbon and such metal materials as Fe, stainless steel(SUS), Nb, Ta, and Mo, is exemplified. Though stainless steel satisfyingany standard can be used as the stainless steel, a standard notcontaining Ni such as SUS430 is further preferred, for the purpose ofbeing able to maintain purity of the melt above and enhancing thermalconductivity.

A size and a shape of a spacer can be selected as appropriate inconsideration of a thickness or the like of the metal coating layer. Forexample, a plate-shaped body or a linear body (wire) is exemplified. Inmaking use of a linear body, a linear body slightly smaller in diameterthan the metal coating layer to be formed may be prepared, and a gap maybe provided between the molded body and the cast by fixing the moldedbody to the cast with this linear body. In this case, most part of thelinear body is buried in the metal coating layer, and hence a compositemember having good appearance is obtained even though the linear bodyremains.

(3) A cast including a surface of contact with the SiC aggregate and athermal expansion portion made of a material higher in coefficient ofthermal expansion than SiC is employed as the cast (in particular, thecomposite member formation portion). The SiC aggregate above is arrangedin this cast, and the thermal expansion portion above expands by heatgenerated at the time of infiltrating this SiC aggregate with molten Mgor Mg alloy above. As a result of this thermal expansion, a gap isproduced between the contact surface of the cast and the SiC aggregate(this gap serves as the unfilled region). Then, molten Mg or Mg alloyabove poured into this gap forms the metal coating layer.

In the technique (1), (2′) above, for example, a powder molded bodyformed with slip casting, a powder molded body formed with pressureforming, a sintered body obtained by further sintering any powder moldedbody above, a sintered body obtained by sintering SiC with which themold was filled with such a method as tapping, as well as a commerciallyavailable sintered body, and the like can be made use of as the moldedbody. In the technique (2), (3) above, any of various molded bodiesdescribed above and a material obtained by tapping the SiC powders abovemay be employed as the SiC aggregate.

In the technique (2), in a case where a spacer is made of a sublimablesubstance such as naphthalene (sublimation temperature: 218° C.), dryice (sublimation temperature: −78.5° C.), and anthracene (sublimationtemperature: 342° C.), deformation of the SiC aggregate due toliquefaction of the spacer can be prevented and in addition a residue(for example, soot or the like) is less likely to remain in the cast,which is preferred.

In addition, when the spacer above is composed of a substance vaporizedat a temperature not higher than a liquidus temperature (melting point)of Mg or the Mg alloy serving as the metal component of the compositemember (substrate), by heating the cast to a temperature not lower thanthe melting point of the spacer and not higher than a solidustemperature of Mg or the Mg alloy above, the spacer above arranged inthe cast can be removed through this heating. Further, in this case, Mgor the Mg alloy above is arranged in a metal placement portion of thecast above, and the SiC aggregate above, the spacer above, and the castin which Mg or the Mg alloy above has been arranged are heated to atemperature not lower than the solidus temperature (melting point) of Mgor the Mg alloy above to thereby melt Mg or the Mg alloy above. Thus,the SiC aggregate arranged on the composite member formation portion ofthe cast above is infiltrated and this heating can be made use of forvaporization and removal of the spacer. In this case, since melting ofMg or the Mg alloy above and removal of the spacer can be performed in asingle heating step, a heating step for removing the spacer does nothave to be provided separately, and thus excellent manufacturability ofthe composite member is achieved. On the other hand, if the heating stepfor removing the spacer is provided separately, the spacer can reliablybe removed and a residue or the like is unlikely. In addition, since aspacer can be selected without taking into account a melting point or aboiling point of a constituent material for the spacer, a degree offreedom in selecting a spacer is enhanced.

A size and a thickness of the spacer above are adjusted such that themetal coating layer having desired size and thickness can be formed onone surface of the substrate. By making use of such a spacer, a metalcoating layer having a uniform thickness is readily formed and acomposite member having a desired dimension can accurately bemanufactured. The spacer above may be arranged such that the metalcoating layer is formed on each of opposing surfaces of the substrate.

Unless excessive vibration is provided to the cast after removal of thespacer above, even the SiC aggregate formed by tapping described abovecan be free-standing, of which shape is kept to such an extent as notcollapsing, and a gap created in a space where the spacer has beenpresent can sufficiently be maintained.

In the technique (3), it is assumed that the cast is formed by combiningdivided pieces, and a cast formed such that each divided piece forming acast main body is made of a material having low coefficient of thermalexpansion α (such as carbon (α: approximately 3.0 to 4.8 ppm/K)) and acoupling member such as a screw or a bolt for coupling the dividedpieces to each other is made of a material higher in coefficient ofthermal expansion α than SiC (α: approximately 3.0 to 6 ppm/K) such asstainless steel can be made use of. In using a coupling member high incoefficient of thermal expansion α, a part of a screw hole or the likeprovided in a bolt, a screw or a cast may be cut in order to avoidbreakage of the cast due to expansion of the coupling member. By thusmaking use of members different in coefficient of thermal expansion,when the cast itself is heated or the cast comes in contact with moltenMg in infiltrating the SiC aggregate with molten Mg or Mg alloy (moltenMg), the cast main body low in coefficient of thermal expansion is lesslikely to experience thermal expansion but the coupling member servingas the thermal expansion portion expands, so that a small gap can beformed between the divided pieces. In addition, since the cast main bodyis less likely to deform by heat (an amount of expansion and shrinkageis small), a composite member having a desired size can accurately beformed.

For example, it is assumed that the SiC aggregate (preform) has acoefficient of thermal expansion α_(S) (ppm/K) and a thickness t_(S)(mm), the cast (divided piece) has a coefficient of thermal expansionα_(M) (ppm/K), the screw coupling the divided pieces of the cast to eachother has a coefficient of thermal expansion α_(N) (ppm/K), a portion ofthe screw buried in the cast (divided piece) has a length t_(N) (mm),the base material metal (magnesium here) has a melting point of 650° C.,and a room temperature before heating the cast is set to 25° C. Here, asthe SiC aggregate is infiltrated with molten Mg and the metal coatinglayer is concurrently formed, an approximate thickness t_(f) (μm) of themetal coating layer formed in the gap between the divided pieces aboveis expressed with Equation (1) below.

t _(f)=(650−25)×(α_(N) ×t _(N)−α_(M)(t _(N) −t _(S))−α_(S) ×t _(S))×10⁻³(μm)  Equation (1)

For example, when it is assumed that α_(S)=3 (ppm/K), t_(S)=4.5 (mm),α_(M)=4 (ppm/K), α_(N)=17.3 (ppm/K), and t_(N)=10 (mm), thickness t_(f)of the metal coating layer is calculated as t_(f)=85 (μm) based onEquation (1) above.

A material for the thermal expansion portion and a length buried arepreferably selected in consideration of the composition of the metalcomponent of the substrate, the SiC content, a material for the cast, athickness of the substrate, and the like, such that the metal coatinglayer has a desired thickness, that is, a desired gap is formed. Forexample, in forming a metal coating layer approximately from 1 μm to 100μm on the substrate having a size of approximately 4.5 mm×100 mm×200 mm,a constituent material different in coefficient of thermal expansionfrom SiC by not less than 1 ppm/K and preferably not less than 3 ppm/Kin consideration of certainty can suitably be made use of as theconstituent material for the coupling member serving as the thermalexpansion portion.

In particular, in forming a composite member having a large size andincluding a metal coating layer, when a gap is provided between the SiCaggregate and the cast as described above so as to allow flow-in ofmolten Mg also in the gap in making a composite above and the metalcoating layer is formed simultaneously with formation of the infiltratedplate, excellent productivity of the composite member is achieved. Here,by performing cooling in one specific direction as described above, ahigh-grade substrate can be formed even though the infiltrated plate hasa large size. Therefore, in the metal coating layer formed on thesurface thereof as well, such a defect as shrinkage at the surface canbe lessened. In particular, even when the metal coating layer is thin,according to the manufacturing method in the present invention in whichcooling in one specific direction is performed, a metal coating layerexcellent in surface property can be formed. In this form, a metalcoating layer having a thickness in accordance with a size of the gapabove can be formed. Therefore, a size of the gap above is preferablyadjusted such that the metal coating layer has a desired thickness.

Meanwhile, in making use of a metal plate as a metal to be present inthe unfilled region, the metal coating layer can be formed as follows.Initially, the metal plate is arranged in the cast (this metal plateserves as the unfilled region). Then, the SiC aggregate above isarranged in the cast where the metal plate above has been arranged. Inmaking a composite of this SiC aggregate and molten Mg or Mg alloy, themetal plate is joined to the SiC aggregate by means of this molten Mg orMg alloy to thereby form the metal coating layer. A metal plate havingdesired composition and desired size and thickness is preferablyprepared as appropriate as the metal plate. In order to further enhanceadhesiveness between the metal plate and the substrate, a low-meltinglayer lower in solidus temperature (melting point) than a constituentmetal of the metal plate may be provided on a surface of the metal plateto be joined to the substrate. In employing the metal plate, the metalcomponent in the substrate and the constituent metal of the metalcoating layer may be different from each other in composition, or themetal coating layer having a smooth surface can readily be formed. Inthis form, any of various molded bodies described above and a materialobtained by tapping SiC powders can be made use of as the SiC aggregateabove.

Alternatively, the composite member according to the present inventionincluding a metal coating layer can be manufactured separately also byjoining a metal plate having a metal coating layer formed after thesubstrate is fabricated. For example, a hot pressing method among themanufacturing methods according to the present invention described abovecan be made use of. In this form, since the substrate made of acomposite material can separately be fabricated, the substrate can befabricated through basic steps without arranging a spacer or a metalplate in a cast or making use of a cast having a special construction,and manufacturability of the substrate is excellent. In addition, thehot pressing method has such various advantages that (1) it can beperformed at a temperature not higher than the melting point of Mg andtherefore a wide choice of constituent materials for the metal coatinglayer is available and thus a composite member in which a metalcomponent of the substrate made of the composite material and aconstituent metal of the metal coating layer are metals different intype can be formed in a simplified manner, (2) since the metal platedeforms along a shape of the surface of the substrate above throughplastic deformation and it is joined in an adhesive manner, jointstrength between the substrate above and the metal coating layer isexcellent, (3) since the metal plate plastically deforms, the metalcoating layer can be formed in spite of presence of a surface defect(such as an external shrinkage cavity) in the substrate above, and inaddition the defect above can be closed and a composite member havingexcellent surface property is obtained, (4) even if there are pores inthe substrate above, the pores can be decreased as they are collapsed bypressurization and thus thermal characteristics of the composite membercan be improved owing to fewer pores, (5) since such an inclusion(solder) as in brazing is not necessary, a composite member excellent inthermal conductivity is obtained, and (6) a metal plate having a smallthickness can be joined and a composite member including a thin metalcoating layer is obtained, and in addition a coefficient of thermalexpansion of the composite member as a whole including the metal coatinglayer can be suppressed because of a small thickness of the metalcoating layer. In manufacturing a substrate, various SiC molded bodiesdescribed above may be made use of or the cast may directly be filledwith SiC powders.

A heating temperature or an applied pressure for a stack of thesubstrate above and the metal plate can be selected as appropriate,depending on composition of a metal component of the substrate,composition of the metal plate, and the like. If the heating temperatureis lower than 300° C. and the applied pressure is lower than 0.5ton/cm², sufficient joint of the stack above is difficult. As theheating temperature above is higher or the applied pressure above ishigher, joint characteristics tend to be excellent. If the heatingtemperature above is not lower than 500° C., sufficient joint can beachieved even though the applied pressure is slightly low. If theheating temperature is too high, however, the metal component in thesubstrate or the metal plate is molten and the substrate or the metalplate deforms or flows out of the gap in a pressure mold. Therefore, theheating temperature is preferably not higher than the solidustemperature (melting point) of the metal component of the substrate orthe metal plate. If the applied pressure is too high, crack of SiCoccurs and hence the applied pressure is preferably approximately nothigher than 9 ton/cm². In addition, if the applied pressure exceeds 5ton/cm², deterioration of the pressure mold is accelerated. Therefore,taking into account life of the pressure mold, it seems that the appliedpressure is further preferably not higher than 5 ton/cm².

In performing hot pressing above, if a joint atmosphere is set to aninert atmosphere, generation of an oxide film on the surface of a metalplate or a substrate can be suppressed. As compared with a case where ajoint atmosphere is set to atmosphere, joint can be achieved at a lowerheating temperature or with a lower applied pressure. For example, an Aratmosphere, a He atmosphere, an N₂ atmosphere, and a vacuum atmosphereare exemplified as the inert atmosphere. In the case of the atmosphere,joint can be achieved through sufficient heating and pressurization, andfacilities can be simplified more than in the case of an inertatmosphere.

When the metal plate above is composed of one metal selected from thegroup consisting of Mg and Al of which purity is not lower than 99% andan alloy mainly composed of Mg or Al, the heating temperature not lowerthan 300° C. can allow sufficient joint of the stack above. Inparticular in the atmosphere, the heating temperature is preferably notlower than 400° C. When the heating temperature is not lower than 400°C. and lower than 500° C., the applied pressure is preferably not lowerthan 5 ton/cm². When the heating temperature is not lower than 500° C.,the applied pressure is preferably not lower than 0.5 ton/cm². In theinert atmosphere, when the heating temperature is not lower than 300° C.and lower than 500° C., the applied pressure is preferably not lowerthan 3 ton/cm². When the heating temperature is not lower than 500° C.,the applied pressure is preferably not lower than 0.5 ton/cm².

When the metal plate above is composed of one metal selected from thegroup consisting of Cu and Ni of which purity is not lower than 99% andan alloy mainly composed of Cu or Ni, the heating temperature not lowerthan 500° C. can allow sufficient joint of the stack above. Inparticular in the atmosphere, the heating temperature is preferably notlower than 600° C. When the heating temperature is not lower than 600°C. and lower than 645° C., the applied pressure is preferably not lowerthan 3 ton/cm². When the heating temperature is not lower than 645° C.,the applied pressure is preferably not lower than 0.5 ton/cm². In theinert atmosphere, when the heating temperature is not lower than 500°C., the applied pressure is preferably not lower than 0.5 ton/cm².

Further, in performing the hot pressing method, by restraining a sidesurface of the substrate made of the composite material as appropriate,deformation of the substrate is suppressed and a composite memberparticularly excellent in dimension accuracy can be produced.Furthermore, upper and lower punching surfaces made use of in the hotpressing method are made as appropriate curved surfaces (a convexsurface and a concave surface), so that prescribed warp can be providedto the substrate above.

In addition to the hot pressing method above, for example, at least onetechnique of brazing, ultrasonic bonding, enveloped casting, rolling(clad rolling), oxide soldering, and joint with an inorganic adhesivecan be made use of as a method of joining a metal plate to a substratemade of a composite material.

<Formation of Large-Sized Composite Member>

<<Cast>>

For example, in fabricating a plate-shaped composite member such as arectangular plate, a cast made use of in infiltration representativelyincludes a bottom surface portion and a sidewall portion erected fromthe bottom surface portion, and a box-shaped cast having an opening on aside opposed to the bottom surface portion can be made use of. Infabricating a composite member in a rectangular plate shape, a boxhaving a parallelepiped shape can suitably be made use of as the cast.An SiC aggregate is accommodated in this box-shaped cast. Afteraccommodation, representatively, the cast is arranged such that a sideof the bottom surface portion of the cast is located on a lower side inthe vertical direction and a side of opening of the cast is located onan upper side in the vertical direction. Then, for example, by makinguse of weight of molten Mg itself, molten Mg flows in from the side ofopening of this cast (a side of supply of molten Mg) toward the side ofthe bottom surface portion (the side opposite to the supply side above)so as to infiltrate the aggregate above therewith. Thus, withoutpressurizing molten Mg or the like, the infiltrated plate can bemanufactured in a simplified manner, for example, at an atmosphericpressure.

Alternatively, infiltration with molten Mg can also be carried out fromthe side of the bottom surface portion of the cast toward the side ofthe upper opening. Namely, the side of the bottom surface portion of thecast can be set as the supply side of molten Mg and the side of openingof the cast can be set as the side opposite to the supply side above.For example, such a construction that a teeming port for molten Mg isprovided at the bottom surface portion of the cast and molten Mg ispushed up from the teeming port by means of a pump, a piston,capillarity, or the like is exemplified. Alternatively, such aconstruction that a cast and a separate vessel are prepared, the bottomsurface portion of the cast and the bottom surface portion of the vesselare connected to each other through a coupling pipe such as a hose,molten Mg is supplied to the vessel, and molten Mg is supplied from theside of the bottom surface portion of the cast by making use of pressureapplied to molten Mg or such a construction as suctioning molten Mg fromthe side of opening of the cast is exemplified. In this form ofsupplying molten Mg from the side of the bottom surface portion of thecast, it is considered that making of a composite proceeds with decreasein volume being compensated for while unsolidified Mg or Mg alloy issupplied from the side of the bottom surface portion of the cast. Inaddition, in this form, it is expected that air in the SiC aggregatereadily escapes because the upper side is open. Moreover, in this form,by cooling the infiltrated plate in one direction from the upper side inthe vertical direction toward the lower side in the vertical direction,a composite member having fewer defects described above can befabricated in spite of an infiltrated plate having a large size. It isconsidered, however, that the form making use of the weight of molten Mgitself above is superior in productivity.

<<Cooling>>

In forming a large-sized composite member in particular, the infiltratedplate above is cooled in one direction from the side of the infiltratedplate opposite to the supply side of molten Mg. For example, in a casewhere the side of supply of molten Mg is defined as the upper side inthe vertical direction and the side opposite to the supply side isdefined as the lower side in the vertical direction, that is, in a caseof supplying molten Mg in the direction of gravity, the infiltratedplate is cooled in one direction from the lower side in the verticaldirection toward the upper side in the vertical direction.Alternatively, in a case where the side of supply of molten Mg isdefined as the lower side in the vertical direction and the sideopposite to the supply side is defined as the upper side in the verticaldirection, that is, in a case of supplying molten Mg in a directionopposite to the direction of gravity, cooling is performed in onedirection from the upper side in the vertical direction toward the lowerside in the vertical direction, that is, in the direction of gravity.More specifically, for example, in a case of manufacturing a compositemember in a rectangular plate shape, with one end side in a longitudinaldirection of the infiltrated plate being defined as the lower side inthe vertical direction and the other end side being defined as the upperside in the vertical direction, cooling is performed from the one endside in the longitudinal direction toward the other end side (or fromthe other end side toward the one end side). For thus cooling theinfiltrated plate in one direction, for example, forced cooling of theside opposite to the side of supply of molten Mg in the infiltratedplate above (hereinafter referred to as a cooling start side) isexemplified. Liquid cooling making use of a liquid coolant, air coolingforcibly sending air, and the like are exemplified as forced cooling. Inusing a coolant such as a liquid coolant, the coolant may be in directcontact with the bottom surface portion of the cast (or the openingportion of the cast) arranged on the cooling start side, or the bottomsurface portion of the cast (or the opening portion of the cast)arranged on the cooling start side may be arranged closer to thecoolant. Alternatively, covering a portion other than a portion toforcibly be cooled, such as the bottom surface portion of the cast, witha heat-insulating material, or moving the cast from the cooling startside, from a high temperature region to a low temperature region, isexemplified. Any cooling method can be made use of and various coolingmethods above may be used as combined.

<<Temperature Gradient>>

In the cooling step above, by cooling the infiltrated plate such that atemperature gradient along the direction of cooling of the infiltratedplate above (for example, a temperature gradient from the lower side inthe vertical direction toward the upper side in the vertical direction)is within a specific range below, an area ratio of the defect portionabove can further be lowered and a higher-grade composite member havingfewer defects can more readily be obtained in spite of the infiltratedplate having a large size. Specifically, temperature difference of aprescribed magnitude is provided between a precedingly cooled one side(for example, the lower side in the vertical direction) and subsequentlycooled the other side (for example, the upper side in the verticaldirection). More specifically, a plurality of temperature measurementpoints are taken along the direction of cooling of the infiltrated plateabove. When each temperature measurement point attains to a prescribedtemperature, temperature difference ΔT_(P) between one temperaturemeasurement point P_(u) and the other temperature measurement pointP_(d) adjacent to a temperature measurement point P_(s) is calculatedand a value ΔT_(P)/1 obtained by dividing this temperature differenceΔT_(P) by a distance 1 between two temperature measurement points P_(u),P_(d) is adopted as a temperature gradient of that temperaturemeasurement point. Here, the infiltrated plate above is cooled such thatthe temperature gradient at each temperature measurement point above isnot less than 0.01° C./mm. As the temperature gradient is greater, thearea ratio of the defect portion above tends to be lower and atemperature gradient not less than 0.1° C./mm and in particular not lessthan 0.5° C./mm is further preferred.

Though any temperature measurement point can be selected, a plurality oftemperature measurement points at regular intervals in the longitudinaldirection of the infiltrated plate above are preferably selected. Inaddition, too small an interval between adjacent temperature measurementpoints makes it difficult to provide sufficient temperature difference,and too large an interval therebetween leads to too great temperaturedifference. Therefore, although depending on a size of the infiltratedplate above in the longitudinal direction, an interval approximatelyfrom 5 to 10 mm seems to be preferred.

<<Cooling Rate>>

In the cooling step above, when cooling is performed such that a rate ofcooling along the direction of cooling of the infiltrated plate above(for example, a rate of cooling from the lower side in the verticaldirection toward the upper side in the vertical direction) is within aspecific range below, an area ratio of the defect portion above canfurther be lowered and a higher-grade composite member having fewerdefects can more readily be obtained in spite of the infiltrated platehaving a large size. Specifically, a plurality of temperaturemeasurement points are taken along the direction of cooling of theinfiltrated plate above. A time period t required for each temperaturemeasurement point to decrease from a prescribed high temperature T_(H)to a prescribed low temperature T_(L) is counted and a value(T_(H)−T_(L))/t calculated by dividing difference T_(H)−T_(L) betweentemperatures T_(H) and T_(L) above by time period t above is defined asa cooling rate at each temperature measurement point above. Here,cooling is performed such that a cooling rate at each temperaturemeasurement point above is not less than 0.5° C./min. As the coolingrate is greater, the area ratio of the defect portion above tends to belower, and the cooling rate not less than 3° C./min, in particular notless than 10° C./min, and further not less than 50° C./min is furtherpreferred. As the cooling rate is greater, a crystal particle of Mg orthe Mg alloy becomes finer and a composite member having good appearanceis obtained.

The temperature gradient or the cooling rate above can be varieddepending on a temperature of the coolant or an amount of the coolant(an amount of blown air or the like) above, a distance between thecoolant and the cast, a state of arrangement of the heat-insulatingmaterial, a moving speed of the cast, or the like. In the case of theconstant temperature gradient above, as the cooling rate above isgreater, the area ratio of the defect portion above is more readilylowered and deformation is less, or in the case of the constant coolingrate above, as the temperature gradient above is greater, the area ratioof the defect portion above is more readily lowered and deformation isless. In addition, if the cooling rate above is high and the temperaturegradient above is great, an extremely high-grade composite member havingvery few defects is readily obtained.

In realizing the temperature gradient or the cooling rate above, directmeasurement of a temperature of molten Mg or Mg alloy with suchtemperature measurement means as a thermocouple will lead to seriousdamage of the temperature measurement means. Therefore, a cast or thelike that can realize the temperature gradient or the cooling rate aboveis preferably prepared for use.

<Pressurization>

In addition, as one step in the composite member manufacturing methoddescribed above, porosity can further be lowered by including thepressurization step of pressurizing the obtained composite solidifiedsubstance so as to collapse pores present in this composite solidifiedsubstance as in the manufacturing method (1-2) described above.

In the pressurization step above, pressurization may be performed atroom temperature. If pressurization is performed while the compositesolidified substance is heated, however, ease in plastic working of themetal component composed of Mg or the Mg alloy can be enhanced and porescan be collapsed with lower pressure. Alternatively, if a pressureduring pressurization is constant, by performing pressurization whilethe composite solidified substance is heated, pores can more effectivelybe collapsed than in a case at room temperature. An applied pressure ispreferably not lower than 1 ton/cm². Since pores are readily collapsedas the applied pressure is greater, a pressure not lower than 3 ton/cm²and further not lower than 5 ton/cm² is preferred. In order to avoiddamage of SiC by pressurization, an applied pressure not higher than 9ton/cm² is preferred. An applied pressure is preferably varied inaccordance with a heating temperature. For example, in a case where aheating temperature is as high as exceeding 240° C., pores caneffectively be collapsed by setting the applied pressure to 1 ton/cm² orhigher. In a case where the heating temperature is set to a relativelylow temperature not lower than 150° C. and not higher than 240° C.,pores can effectively be collapsed by setting the applied pressure to 3ton/cm² or higher. In a case where the heating temperature is set to atemperature not lower than room temperature and lower than 150° C.,pores can effectively be collapsed by setting the applied pressure to 5ton/cm² or higher. Though pores are readily collapsed as the temperatureis higher as described above, in order not to melt solidified Mg or Mgalloy, the upper limit of the heating temperature is set to the meltingpoint (liquidus temperature) of Mg or the Mg alloy in the compositemember. In particular, when the heating temperature is not lower than600° C., pores can sufficiently be decreased even though the appliedpressure is slightly low. A heating temperature and an applied pressureare preferably selected as appropriate in such a range that thecomposite solidified substance is not broken or excessive strain is notintroduced. Taking into account prevention of breakage or lowering instrain, the heating temperature is preferably not lower than 300° C. andin particular not lower than 400° C. and not higher than 600° C. Forexample, pressurization with pressure not lower than 1 ton/cm² whileheating the substrate above or the composite member including the metalcoating layer above to a temperature not lower than 300° C. and lowerthan a solidus temperature (melting point) of the metal component of thesubstrate and the constituent metal of the metal coating layer isexemplified. If a pressurization time period is too long, strain or thelike is applied to Mg or the Mg alloy in the composite member above andhence the pressurization time period for approximately 10 minutes ispreferred. If several mass % Li is added to magnesium, a crystalstructure of magnesium becomes a bcc structure which is readilyplastically deformed, and pores can effectively be collapsed with arelatively low pressure not higher than 5 ton/cm² even at a temperaturefrom room temperature to less than 150° C.

After the pressurization step, in particular for removing strain causedin the metal component (Mg or the Mg alloy) in the composite memberabove, annealing may be performed. The upper limit of an annealingtemperature is set to a melting point (liquidus temperature) of themetal component (Mg or the Mg alloy) in the composite member.

A method of manufacturing the composite solidified substance above whichis an object to be pressurized is not particularly specified. Acomposite solidified substance manufactured with the manufacturingmethod (1-1) according to the present invention as described above or aproduct manufactured with the infiltration method with the use of SiCnot having an oxide film formed as a raw material may be adopted.Alternatively, a composite member low in porosity is obtained byapplying the pressurization step above to a product manufactured with apowder metallurgy method in which many pores are generally present (amethod of mixing powders of a base material metal and raw materialpowders of a non-metal inorganic material such as SiC and molding andfiring the powder mixture) or to a product manufactured with meltingmethod (a method of solidifying a melt mixture in which a raw materialof a non-metal inorganic material such as SiC is mixed in a moltenmetal).

The pressurization step above or the step of removing strain isapplicable not only to a form including only a substrate composed of acomposite material but also to a form including a metal coating layer onat least one surface of the substrate. In a case of forming a metalcoating layer with the hot pressing method described above as well, itis expected that pores can further be decreased by further adding thepressurization step. By including such a pressurization step, a densesubstrate having porosity not higher than 5% and in particular lowerthan 3% or a composite member including a metal coating layer on a densesubstrate is obtained, and variation in characteristics such as thermalcharacteristics as described above can be lowered.

Effects of the Invention

The composite member according to the present invention in the firstform has excellent thermal characteristics because a content of a phaselow in thermal conductivity derived from use of an infiltration agent orporosity is low. According to the manufacturing method (1-1), (1-2) inthe present invention, a composite member low in porosity can bemanufactured without raising an infiltration temperature to a very hightemperature or without using an infiltration agent.

The composite member according to the present invention in the secondform is excellent not only in adaptability in coefficient of thermalexpansion to a semiconductor element and the like but also in thermalconductivity. According to the composite member manufacturing method inthe present invention (the sintering method), the composite memberaccording to the present invention excellent in thermal characteristicsabove can be manufactured with high productivity.

Plating by electroplating can be applied to the composite memberaccording to the present invention in the third form by including themetal coating layer. According to the composite member manufacturingmethod in the present invention (the composite integration method, thehot pressing method), the composite member including the metal coatinglayer above can be manufactured.

The composite member according to the present invention in the fourthform is of high grade in spite of its large size, because defects arenot present in a concentrated manner. According to the composite membermanufacturing method in the present invention (the one-direction coolingmethod), a composite member of high grade in spite of its large size canbe manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph (×50 magnification) of a composite memberfabricated in a Test Example I-1, (I) showing a sample No. I-1-1, (II)showing a sample No. I-1-2, (III) showing a sample No. I-1-3, and (IV)showing a sample No. I-1-4.

FIG. 2 is a graph showing relation between a heating temperature insubjecting raw material SiC to oxidation treatment and porosity of anobtained composite member, with regard to a sample in which SiC powdersof #120:#600=8:2 were adopted as a raw material, among samplesfabricated in Test Example I-1.

FIG. 3 is a micrograph (×50 magnification) of a composite memberfabricated in a Test Example I-2, (I) showing a sample No. I-2-9 and(II) showing a sample No. I-2-16.

FIG. 4 is a micrograph (×50 or ×500 magnification) of a cross-section ofa composite member in an Embodiment II-1, (I) showing an example where anetwork portion is thick (×50 magnification) and (II) showing an examplewhere a network portion is thin (×500 magnification).

FIG. 5 is a micrograph (×50 magnification) of a cross-section of acomposite member in an Embodiment II-2.

FIG. 6 is a micrograph (×100 magnification) of a composite memberfabricated in a Test Example III-1.

FIG. 7 is a schematic construction diagram of a cast used in TestExample III-1, (A) showing an exploded perspective view and (B) showinga front view.

FIG. 8 is an illustrative diagram illustrating a state where a metal toform an SiC aggregate and a metal component of a substrate is arrangedin the cast used in Test Example III-1.

FIG. 9 is an exploded perspective view showing an overview of a castused in fabricating a composite member in a fourth form.

FIG. 10 is a micrograph (×50 magnification) of a cross-section of thecomposite member in the fourth form, (I) showing a composite member inan Embodiment IV-1 and (II) showing a composite member in an EmbodimentIV-2.

FIG. 11 is an illustrative diagram showing a state where the cast usedfor fabricating the composite member in the fourth form was loaded in anatmosphere furnace.

FIG. 12 is an illustrative diagram showing a state where the cast iscooled by water-cooled copper in the fourth form, (I) showing an examplewhere a heat-insulating material is not arranged in contact with thecast and (II) showing an example where the cast is covered with theheat-insulating material.

FIG. 13 is an illustrative diagram showing a state in the fourth formwhere the cast is moved from a high-temperature region to alow-temperature region for cooling.

MODES FOR CARRYING OUT THE INVENTION

In the drawings below, the same reference character refers to an elementhaving the same denotation.

First Form: Lowering in Porosity Test Example I-1

A composite member made of a composite of pure magnesium or a magnesiumalloy and SiC was fabricated, and porosity and thermal characteristicswere examined.

A composite member was fabricated as follows. A base material shown inTable 1 (Mg: pure magnesium composed of 99.8 mass % or more Mg and animpurity (denoted as “Mg” in Table 1), AZ31 and AZ91: magnesium alloycontaining Al, Zn), SiC powders different in average particle size(average particle size: #120 (approximately 150 μm), #600 (approximately25 μm), #1000 (approximately 15 μm), and an SiC sintered body wereprepared as raw materials. All raw materials used were commerciallyavailable.

The raw material SiC powders and the SiC sintered body were subjected tooxidation treatment at a temperature (° C.) shown in Table 1. A heatingtime period was set to 2 hours for any sample. After the oxidationtreatment, an amount of oxide film was measured. Table 1 shows theresults. The amount of oxide film was measured with an ICP-AES(inductively coupled plasma atomic emission spectroscopy) apparatus. Itis noted that a sample not subjected to oxidation treatment is denotedas “No Oxidation” in Table 1.

A mixture in which coated SiC powders, a coated SiC sintered body thathad been subjected to oxidation treatment, and untreated SiC powderswere mixed at a mixture ratio (volume ratio) shown in Table 1 was pouredinto a cast (50 mm×30 mm×6 mm) in a parallelepiped shape, to whichvibration was applied subsequently as appropriate for leveling (tapfilling). Then, a molten metal to serve as the base material wasintroduced in the cast and the cast was held at an infiltrationtemperature (° C.) for an infiltration time period (hour) shown in Table1, so that the SiC aggregate in the cast was infiltrated with the moltenmetal. After the infiltration time period above elapsed, the cast wascooled so as to solidify the molten metal, to thereby obtain thecomposite member (the composite solidified substance). It is noted thata process from introduction to cooling of the molten metal was performedin an Ar atmosphere (atmospheric pressure).

Porosity (%), thermal conductivity (W/K·m), a coefficient of thermalexpansion (ppm/K), and an SiC content (volume ratio, %) of each obtainedcomposite member were examined. Table 1 shows the results.

Porosity (%) in the composite member was found as follows. Initially,the obtained composite member was buried in a resin and fixed theretosuch that SiC present in a region not impregnated with a metal component(pure magnesium or the magnesium alloy) does not become free. Anycross-section of the composite member buried in the resin was polishedwith a barrel polishing plate by using commercially availablenon-corrosive diamond paste and lubricant, and thereafter thiscross-section was observed with an optical microscope (magnification:×50). In this observed image, a region not smaller than 2 mm×3 mm wassubjected to image processing with a commercially available imageprocessing apparatus so as to find a total area of all pores present inthe region above. A value of {(total area)/(area of the region)}×100 wascalculated as porosity in this cross-section, and an average value ofporosity of the cross-section for n=3 is shown in Table 1.

Thermal conductivity and a coefficient of thermal expansion of thecomposite member were measured by cutting a test piece from the obtainedcomposite member and by using a commercially available measuringinstrument. It is noted that a coefficient of thermal expansion wasmeasured in a range from 30° C. to 150° C. In addition, a sample ofwhich thermal characteristics could not be measured due to failure inobtaining a test piece for such reasons as insufficient impregnationafter lapse of the infiltration time period is denoted as “MeasurementNot Available” in Table 1.

The SiC content in the composite member was found as follows. Anycross-section of the composite member was observed with an opticalmicroscope (magnification: ×50), this observed image was subjected toimage processing with a commercially available image processingapparatus, a total area of all SiC present in this cross-section wasfound, a value obtained by converting this total area to a volume ratiowas adopted as a volume ratio based on this cross-section, and anaverage value of the volume ratio of the cross-section for n=3 is shownin Table 1.

TABLE 1 Raw Material SiC Mixture Oxidation Molten Metal Molten MetalRatio Treatment Infiltration Infiltration Sample Base Material #600Temperature Temperature Time Period No. Composition #120 #1000 (° C.) (°C.) (hour) I-1-1 Mg 8 2 No Oxidation 875 2 I-1-2 Mg 8 2 600 875 2 I-1-3Mg 8 2 875 875 2 I-1-4 Mg 8 2 1000  875 2 I-1-5 Mg 8 2 875 750 2 I-1-6AZ31 8 2 875 750 2 I-1-7 AZ91 8 2 875 750 2 I-1-8 Mg 8 2 875 700 2 I-1-9AZ31 8 2 875 700 2 I-1-10 AZ91 8 2 875 700 2 I-1-11 AZ31 8 2 875 875 2I-1-12 AZ91 8 2 875 875 2 I-1-13 Mg 8 2 No Oxidation 1000 2 I-1-14 Mg 82 875 1000 2 I-1-15 AZ31 8 2 875 1000 2 I-1-16 AZ91 8 2 875 1000 2I-1-17 Mg 7   3^(#10) 875 875 2 I-1-18 Mg 7   3^(#10) 875 825 2 I-1-19Mg 7   3^(#10) 875 775 2 I-1-20 Mg 7   3^(#10) 875 725 2 I-1-21 Mg 7  3^(#10) 875 675 2 I-1-22 Mg 7   3^(#10) 875 675 6 I-1-23 Mg 7  3^(#10) 1000  675 2 I-1-24 Mg Sintered Body 875 875 2 I-1-25 MgSintered Body 875 875 2 Coefficient of Thermal SiC Thermal ExpansionVolume Oxide Film in Sample Conductivity 30-150° C. Ratio Coated SiCPorosity No. (W/K · m) (ppm/K) (%) (mass %) (%) I-1-1 168 9.4 59 0.189.5 I-1-2 216 8.6 60 0.21 6.2 I-1-3 225 9.7 58 0.45 0.9 I-1-4 235 9.7 580.86 0.4 I-1-5 163 9.3 58 0.45 12.1 I-1-6 169 8.4 60 0.45 2.5 I-1-7 1288.1 59 0.45 2.9 I-1-8 Not Impregnated 58 0.45 Measurement Not AvailableI-1-9 151 8.8 58 0.45 11.2 I-1-10 131 8.2 59 0.45 2.4 I-1-11 174 7.9 610.45 2.7 I-1-12 129 8.1 61 0.45 2.1 I-1-13 220 9.2 65 0.18 5.0 I-1-14223 9.1 58 0.45 4.7 I-1-15 173 8.8 58 0.45 0.8 I-1-16 157 7.9 59 0.450.6 I-1-17 228 7.8 70 0.66 0.8 I-1-18 234 7.6 69 0.66 0.6 I-1-19 216 8.270 0.66 0.9 I-1-20 223 7.7 70 0.66 1.2 I-1-21 208 7.8 69 0.66 8.5 I-1-22224 7.6 69 0.66 1.4 I-1-23 220 7.7 70 1.18 1.3 I-1-24 238 4.6 80 0.461.1 I-1-25 239 4.4 80 0.46 1.2 x^(#10): Sample containing #1000 SiCpowders; and x: Sample containing #600 SiC powders (x = 2 or 3)

As shown in Table 1, it can be seen that the composite member of whichporosity is lower than 3% is higher in thermal conductivity than othersidentical in composition of the base material. In addition, it can beseen that such a composite member can be manufactured by performingoxidation treatment at a heating temperature not lower than 700° C.,employing coated SiC powders having an oxide film not lower than 0.4mass % with respect to SiC formed, and adjusting as appropriate aninfiltration temperature in accordance with composition of a basematerial. Moreover, it can be seen that the amount of oxide film can beincreased to an amount exceeding 0.65 mass % by decreasing an averageparticle size of the raw material SiC powders, increasing an amount ofuse of small SiC powders, and raising a temperature for oxidationtreatment. Then, it can be seen that, by setting the amount of oxidefilm to an amount exceeding 0.65 mass %, a composite member lower inporosity and higher in thermal conductivity than in a case where theamount of oxide film is set to 0.45 mass % can be manufactured even ifthe infiltration temperature is low.

In the micrograph in FIG. 1, a portion that looks black representspores, and a plurality of particulate substances that look rectangularrepresent SiC (to be understood similarly also in FIG. 3 which will bedescribed later). As shown in FIG. 1, it can be seen that pores insamples Nos. I-1-3 and I-1-4 manufactured with the coated SiC powderswere considerably decreased as compared with sample No. I-1-1manufactured with untreated SiC powders. In particular, it can be seenthat a large number of large pores are observed in sample No. I-1-1,whereas there are fewer large pores in sample No. I-1-3 (the amount ofoxide film: 0.45 mass %). It can be seen that there are many pores alsoin sample No. I-1-2 (the amount of oxide film: 0.21 mass %) for which aheating temperature was set to a temperature lower than 700° C. Insample No. I-1-4 (the amount of oxide film: 0.86 mass %) for which theheating temperature for oxidation treatment was further raised, porescannot substantially be observed. It can be seen from these photographsin FIG. 1 and the graph in FIG. 2 that pores can considerably bedecreased if the heating temperature for oxidation treatment is notlower than 700° C. and in particular not lower than 800° C. and theamount of oxide film is not lower than 0.4 mass %. In addition, it canbe seen that pores can sufficiently be decreased even though the amountof oxide film is not higher than 1 mass %.

Further, a cross-section of each sample of the composite member of whichporosity was lower than 3%, manufactured with the coated SiC powdershaving the oxide film formed with the heating temperature being set to700° C. or higher, was analyzed with an Auger electron spectroscope.Consequently, oxygen was observed in the SiC outer peripheral region inthe central region (oxygen) of each sample, and concentration of thisoxygen was higher than in the main region with respect to this SiC. Thisfact supports that the oxide film formed on raw material SiC and themolten metal reacted to each other and contact between SiC and themolten metal was promoted. It is noted that concentration of oxygen wascompared by selecting SiC in the central region (oxygen) and byselecting any five measurement points in the outer peripheral regionwith respect to this SiC (a point at a distance by 150 nm outward fromthe contour line of SiC) and in the main region, and concentration ofoxygen at all five points in the SiC outer peripheral region was higherthan in the main region.

Furthermore, among composite members manufactured with the coated SiCpowders on which the oxide film was formed with the heating temperaturebeing set to 700° C. or higher, a composite member achieving highthermal conductivity not lower than 180 W/K·m was obtained and thetendency is such that thermal conductivity is improved as porosity islower. In addition, the composite member of which thermal conductivityis not lower than 180 W/K·m and coefficient of thermal expansion is from4 to 10 ppm/K is expected to suitably be made use of as a heat radiationmember, because it has thermal characteristics required in a heatradiation member of a semiconductor element.

Additionally, it can be seen that porosity can be lowered by setting aninfiltration temperature to 675° C. or higher, in manufacturing acomposite member with the use of coated SiC powders. If the infiltrationtemperature was set to 1000° C. in a case where the metal component inthe composite member was pure magnesium, porosity exceeded 3% in somecases. This may be because of generation of shrinkage cavities or gasholes, and it seems that improvement can be achieved by taking suchmeasures as retaining a molten state for a sufficient period of time ata temperature not higher than 875° C. and performing dehydrogenationbefore the solidification step.

Test Example I-2

A composite solidified substance made of a composite of pure magnesiumand SiC was subjected to pressurization treatment to thereby fabricate acomposite member, and porosity thereof was examined

In this test, a plurality of composite solidified substances werefabricated under the conditions similar to those for the compositesolidified substance represented as sample No. I-1-1 fabricated in TestExample I-1 (a substance fabricated with the use of untreated SiCpowders), and each composite solidified substance was subjected to thefollowing pressurization treatment.

The fabricated composite solidified substance was subjected topressurization treatment under such conditions as 1 to 5 ton/cm² and 10minutes while it was held at a temperature from room temperature (30°C.) to 600° C., to thereby fabricate the composite member. Manufacturingconditions and porosity of each sample are shown in Table 2.

Porosity (%) of the obtained composite member was measured as in TestExample I-1. Table 2 shows the results. In addition, an SiC content inthe obtained composite member was found as in Test Example I-1, and itranged from 58 to 60 volume % in any sample.

TABLE 2 Raw Material SiC Pressurization Treatment Sample Base MaterialMixture Ratio Oxidation Treatment Molten Metal Infiltration TemperaturePressure Porosity No. Composition #120 #600 Temperature (° C.)Temperature (° C.) (° C.) (t/cm²) (%) I-1-1 Mg 8 2 No Oxidation 875 — —9.9 I-2-1 Mg 8 2 No Oxidation 875 30 1 9.9 I-2-2 Mg 8 2 No Oxidation 87530 3 9.1 I-2-3 Mg 8 2 No Oxidation 875 30 5 5.0 I-2-4 Mg 8 2 NoOxidation 875 150 1 9.4 I-2-5 Mg 8 2 No Oxidation 875 150 3 5.1 I-2-6 Mg8 2 No Oxidation 875 150 5 3.5 I-2-7 Mg 8 2 No Oxidation 875 240 1 7.5I-2-8 Mg 8 2 No Oxidation 875 240 3 4.4 I-2-9 Mg 8 2 No Oxidation 875240 5 1.2 I-2-10 Mg 8 2 No Oxidation 875 400 1 5.6 I-2-11 Mg 8 2 NoOxidation 875 400 3 4.6 I-2-12 Mg 8 2 No Oxidation 875 400 5 1.2 I-2-13Mg 8 2 No Oxidation 875 500 1 4.3 I-2-14 Mg 8 2 No Oxidation 875 500 33.3 I-2-15 Mg 8 2 No Oxidation 875 500 5 0.7 I-2-16 Mg 8 2 No Oxidation875 600 5 0.6

As shown in FIG. 3, it can be seen that large pores can be eliminated byperforming pressurization treatment under a specific condition.Specifically, as shown in Table 2, it can be seen that porosity of thecomposite member significantly lowers by setting an applied pressure to5 ton/cm² or higher at room temperature, by setting an applied pressureto 3 t/cm² or higher at a heating temperature of 150° C., and by settingan applied pressure to 1 t/cm² or higher at a heating temperature notlower than 240° C. In addition, it can be seen that porosity caneffectively be lowered as the heating temperature at the time ofpressurization is higher and the applied pressure is higher. Thecomposite member low in porosity thus fabricated is considered toexhibit high thermal conduction characteristics as in Test Example I-1.It can be seen from this test that a composite member low in porosity isobtained by performing specific pressurization treatment after treatmentof infiltration with the molten metal without subjecting raw materialSiC to specific treatment. Moreover, it is expected that porosity caneffectively be lowered if the composite solidified substance in whichraw material SiC has been subjected to specific treatment is subjectedto the specific pressurization treatment above.

Second Form: Formation of Network Portion Embodiment II-1 Test ExampleII-1

A substrate (composite member) made of a composite material made of acomposite of pure magnesium and SiC was fabricated and thermalcharacteristics thereof were examined.

An ingot (a commercially available product) of pure magnesium composedof 99.8 mass % or more Mg and an impurity and a commercially availableSiC sintered body (relative density of 80%, 200 mm long×100 mm wide×5 mmthick) were prepared as raw materials.

The prepared SiC sintered body was subjected to oxidation treatment at875° C.×2 hours so as to form an oxide film, so that wettability betweenmolten pure magnesium and the SiC sintered body was enhanced. The stepof oxidation treatment above may not be performed.

The composite member was formed by accommodating the SiC sintered bodyabove in a cast, infiltrating the sintered body with molten puremagnesium, and solidifying pure magnesium.

The cast above is made of carbon, it is a box in a parallelepiped shapeopening on one side, and it is integrally formed by combining aplurality of divided pieces. An internal space of this cast is made useof as a space for accommodating the sintered body. Here, the internalspace of the cast was made to have a size conforming to the sinteredbody above, so that there was substantially no gap provided between thesintered body and the cast when the sintered body was accommodated inthe cast. It is noted that an integrally molded cast may be made use ofinstead of a construction formed by combining divided pieces.

In addition, here, the sintered body above was accommodated in the castafter a commercially available release agent was applied to a portion ofcontact of an inner surface of the cast with the sintered body. Releaseof the composite member can be facilitated by applying the releaseagent. The step of applying this release agent may not be performed.This matter relating to the release agent is similarly applicable alsoto the first form described above and each form which will be describedlater.

The cast above has an ingot placement portion coupled to a periphery ofan opening portion, on which the prepared ingot above is arranged. Byheating this cast to a prescribed temperature, the ingot is molten.Heating of the cast is carried out by loading the cast in an atmospherefurnace capable of heating.

Here, the atmosphere furnace above was adjusted such that theinfiltration temperature was set to 775° C., an Ar atmosphere was set,and a pressure of the atmosphere was set to an atmospheric pressure.Molten pure magnesium flowed into the internal space of the cast throughthe opening portion of the cast and the sintered body arranged in theinternal space was infiltrated therewith. After infiltration, the castwas cooled to solidify pure magnesium. Here, a bottom portion side waspositively cooled such that cooling was achieved in one direction fromthe bottom portion of the cast toward the opening portion. By carryingout cooling as such, internal defects can be reduced even in alarge-sized composite member, and a high-quality composite member isobtained. In a case of a small-sized composite member, a high-qualitycomposite member is obtained without carrying out cooling in onedirection as described above.

The composite member of 200 mm long×100 mm wide×5 mm thick was obtainedwith the use of the cast above. Components of the obtained compositemember were examined with the use of an EDX apparatus. Then, thecomponents were Mg and SiC and the remainder of inevitable impurities,that were the same as in the used raw materials. In addition, theobtained composite member was subjected to a CP (Cross-section Polisher)process to expose a cross-section, that was examined in SEM observation.Then, SiC was directly bonded to each other. Namely, such a porous bodythat a network portion was formed of SiC was obtained, that was the sameas in the used raw material sintered body. In addition, thecross-section of the obtained composite member was observed with anoptical microscope (×50 magnification or ×500 magnification), Then, itcould be confirmed that a gap between SiC and SiC was infiltrated withpure magnesium as shown in FIGS. 4(I) and 4(II). A portion forming acontinuous web shape in FIG. 4(I) represents SiC, while a portionclustered like a particle represents pure magnesium. It can be seen thatthis composite member has a thick network portion. A portion of lightcolor in FIG. 4(II) represents SiC (including a large mass), while aportion of dark color represents pure magnesium. It can be seen thatthis composite member has a thin network portion.

An SiC content in each obtained composite member was measured. Then, theSiC content was 80 volume % in each composite member. The SiC contentwas found in such a manner that any cross-section of the compositemember was observed with an optical microscope (×50 magnification), theobserved image was subjected to image processing with a commerciallyavailable image analysis apparatus, a total area of SiC in thiscross-section was found, a value obtained by converting this total areainto a volume ratio was adopted as a volume ratio based on thiscross-section (area ratio≈volume ratio), a volume ratio in thecross-section for n=3 was found, and an average value thereof wascalculated.

Coefficient of thermal expansion α (ppm/K) and thermal conductivity κ(W/m·K) of each obtained composite member were measured. Then, thecomposite member having a thick network portion had coefficient ofthermal expansion α: 4.0 ppm/K and thermal conductivity κ: 301 W/m·K,and the composite member having a thin network portion had coefficientof thermal expansion α: 4.4 ppm/K and thermal conductivity κ: 270 W/m·K.The coefficient of thermal expansion and the thermal conductivity weremeasured by cutting a test piece from the obtained composite member andby using a commercially available measuring instrument. The coefficientof thermal expansion was measured in a range from 30° C. to 150° C.

From the foregoing, the obtained composite member has excellentadaptability to a semiconductor element or peripherals thereof having acoefficient of thermal expansion around 4 ppm/K and also high thermalconductivity. Therefore, the composite member is expected to suitably bemade use of as a constituent material for a heat radiation member of thesemiconductor element above.

Test Example II-2

SiC aggregates were fabricated under various conditions, and the SiCaggregates were infiltrated with pure magnesium to thereby fabricatecomposite members. Thermal characteristics of the obtained compositemembers were examined.

An ingot (a commercially available product) of pure magnesium composedof 99.8 mass % or more Mg and an impurity and commercially available SiCpowders (particle size from 10 to 170 μm, average particle size: 120 μm)were prepared as raw materials. Then, SiC aggregates (200 mm long×100 mmwide×5 mm thick) were fabricated under the conditions shown in Table 3and infiltrated with molten pure magnesium under the conditions the sameas in Test Example II-1, followed by solidification.

TABLE 3 Network Portion Forming Method Sintering Network Portion SampleMolding Temperature Presence/ Constituent No. Method Method Atmosphere °C. Absence Material II-1 CIP — — — Absent — II-2 CIP — — — Absent — II-3CIP — — — Absent — II-4 CIP — — — Absent — II-5 CIP Sintering Vacuum1800 Present SiC II-6 CIP Sintering Vacuum 1800 Present SiC II-7 CIPSintering Vacuum 2200 Present SiC II-8 CIP Sintering Vacuum 2200 PresentSiC II-9 CIP Sintering Vacuum 2200 Present SiC II-10 Tapping — — —Absent — II-11 Tapping — — — Absent — II-12 Tapping Sintering Vacuum2200 Present SiC II-13 Tapping Sintering Vacuum 1800 Present SiC II-14Tapping Sol-Gel — — Present SiC II-15 Tapping Sintering Vacuum 2200Present SiC II-16 Tapping Sintering (Si) Nitrogen 1600 Present Si₄N₃II-17 Tapping Sintering (Si) Nitrogen 1600 Present Si₄N₃ II-18 SlipCasting Sintering Vacuum 1800 Present SiC II-19 Dry Pressing SinteringVacuum 1800 Present SiC II-20 Wet Pressing Sintering Vacuum 1800 PresentSiC II-21 Doctor Blade Sintering Vacuum 1800 Present SiC II-22 SlipCasting — — — Absent — II-23 Dry Pressing — — — Absent — II-24 WetPressing — — — Absent — II-25 Doctor Blade — — — Absent — Rate ofThermal Thermal Sample Network Portion Oxidation SiC Amount ConductivityExpansion No. Thickness Treatment Volume % W/mK ppm/K II-1 — Yes 70.6219 7.4 II-2 — No 70.7 201 7.3 II-3 — Yes 79.7 253 5.7 II-4 — Yes 86.3264 4.9 II-5 Thin Yes 80.3 270 4.4 II-6 Thin No 79.7 241 4.3 II-7 ThickYes 79.8 301 4.0 II-8 Thick No 79.9 274 4.0 II-9 Thick Yes 85.7 318 3.7II-10 — Yes 59.8 207 9.4 II-11 — Yes 64.7 211 8.5 II-12 Thick Yes 59.7224 5.0 II-13 Thin Yes 59.8 219 7.1 II-14 Thin Yes 59.8 217 7.2 II-15Thick Yes 64.6 232 4.6 II-16 Thick Yes 60.4 201 4.9 II-17 Thick Yes 64.7205 4.5 II-18 Thin Yes 80.1 268 4.4 II-19 Thin Yes 79.6 269 4.5 II-20Thin Yes 80.1 271 4.4 II-21 Thin Yes 80.3 272 4.3 II-22 — Yes 70.0 2177.2 II-23 — Yes 70.2 220 7.5 II-24 — Yes 70.4 221 7.4 II-25 — Yes 70.1219 7.3

Regarding the sample for which “tapping” was adopted as a moldingmethod, a powder molded body was fabricated with the SiC powders above.Regarding the samples indicated with “Oxidation Treatment: Yes” amongthe samples above, an SiC aggregate was fabricated with the SiC powdersincluding the oxide film, that were obtained by subjecting the SiCpowders above to oxidation treatment at 1000° C.×2 hours.

Regarding the samples for which “CIP” or “dry pressing” was adopted as amolding method, a powder molded body was fabricated under knownconditions, with the use of the SiC powders above. Regarding the samplesfor which “wet pressing” or “doctor blade” was adopted as a moldingmethod, in addition to the SiC powders above, water was used in wetpressing and an organic solvent was used in the doctor blade method, tothereby fabricate a powder molded body under known conditions.

Regarding the samples for which “slip casting” was adopted as a moldingmethod, slurry was fabricated by preparing SiC powders, a surfactant andwater, setting a volume ratio between water and SiC powders to water:SiC powders≈5:5, and then adding the surfactant. Here, slurry including20 mass % urea aqueous solution (with the entire slurry being assumed as100 mass %) was prepared. Alternatively, slurry or the like including acommercially available polycarboxylic-acid-based aqueous solution may bemade use of. The fabricated slurry was poured into the cast followed byair drying, to thereby fabricate a powder molded body.

The sample for which “sintering” was adopted as a method of forming anetwork portion was obtained by sintering the fabricated powder moldedbody above under the conditions in Table 3. The sample for which“sintering (Si)” was adopted as a method of forming a network portionwas obtained by fabricating a powder molded body with the use of apowder mixture in which SiC powders and Si powders (average particlesize: 0.1 μm) were mixed and by sintering this powder molded body underthe conditions in Table 3.

Regarding the sample for which “sol-gel” was adopted as a method offorming a network portion, a solution of polycarbosilane was prepared, afabricated powder molded body was infiltrated therewith, and thereafterit was heated to 800° C.

Components of each obtained composite member were examined with the useof an EDX apparatus. Then, the samples except for samples Nos. II-16 andII-17 contained the components of Mg and SiC and the remainder ofinevitable impurities, that were the same as in the used raw materials.In samples Nos. II-16 and II-17, Si₄N₃ was present between SiC and SiC.In addition, the obtained composite member was observed with an SEM asin Test Example II-1. Then, in any of the sample sintered at atemperature not lower than 1800° C., the sample sintered in a nitrogenatmosphere, and the sample for which the sol-gel method was made use of,a network portion bonding SiC to each other was present and each samplehad a porous body. On the other hand, the samples for which sinteringwas not performed or the sol-gel method was not adopted were all in sucha state that SiC particles were dispersed randomly in a base material ofpure magnesium.

An SiC content, coefficient of thermal expansion α (ppm/K) and thermalconductivity κ (W/m·K) of each obtained composite member were measuredas in Test Example II-1. Table 3 shows the results.

As shown in Table 3, it can be seen that each sample of which SiCcontent exceeds 70 volume % is low in coefficient of thermal expansionand satisfies 4 ppm/K to 8 ppm/K. In addition, it can be seen that thesample having a network portion in spite of its SiC content being in arange from 50 volume % to 70 volume % is low in coefficient of thermalexpansion and satisfies 4 ppm/K to 8 ppm/K. Moreover, it can be seenthat the coefficient of thermal expansion becomes lower as the SiCcontent increases. Additionally, it can be seen that each samplesatisfying a coefficient of thermal expansion in a range from 4 ppm/K to8 ppm/K is high in thermal conductivity, that is, not lower than 180W/m·K.

Further, the following can be seen from Table 3, based on comparisonbetween the samples of which SiC content is substantially the same.

(1) A sample for which sintering was performed has a low coefficient ofthermal expansion and high thermal conductivity.

(2) Thermal conductivity can be improved by making use of an SiCaggregate including an oxide film.

(3) A sample in which SiC is directly bonded to each other, that is, asample having a network portion composed of SiC, is higher in thermalconductivity than a sample having a network portion composed of anon-metal inorganic material other than SiC.

Furthermore, it can be seen that the composite member of which SiCcontent exceeds 70 volume % is obtained by using any one of slipcasting, pressure forming and the doctor blade method.

In addition, regarding the sample having a network portion, the samplehaving a thick network portion as shown in FIG. 4(I) and the samplehaving a thin network portion as shown in FIG. 4(II) were obtained.Then, the sample having the network portion was subjected to a CPprocess to expose a cross-section, that was observed with an SEM (×50magnification here). In this cross-section, any line segment of 1 mm wastaken with respect to an actual dimension of each sample, and the numberof intersections between the contour line of the SiC aggregate in thecomposite member and the line segment above was counted. Here, anaverage for n=5 (n: the number of line segments) was calculated, thesample of which average number of intersections was not greater than 50was defined as having a thick network portion, and the sample of whichaverage number of intersections exceeded 50 was defined as having a thinnetwork portion. Table 3 shows the results. It is noted thatmagnification of the cross-section can be adjusted as appropriate forfacilitating observation.

As shown in Table 3, it can be seen that the network portion tends to bethick when the sintering temperature is not lower than 2000° C. Forexample, sample No. II-5 and sample No. II-7 manufactured under the sameconditions except for difference in sintering temperature were comparedwith each other. Then, in sample No. II-5 for which the sinteringtemperature was lower than 2000° C., the number of intersections abovewas 224 (n=a value from 1 to 5: 130 to 320), and in sample No. II-7 forwhich the sintering temperature was not lower than 2000° C., the numberof intersections above was 11 (n=a value from 1 to 5: 8 to 14).

In addition, as shown in Table 3, it can be seen that, if the SiCcontent is substantially the same, the sample having a thick networkportion is lower in coefficient of thermal expansion and higher inthermal conductivity than the sample having a thin network portion andthus excellent in thermal characteristics. On the other hand, the samplehaving a thin network portion is excellent in mechanicalcharacteristics. For example, comparing sample No. II-5 and sample No.II-7 above with each other, sample No. II-5 having a thin networkportion had a coefficient of elasticity of 270 GPa and tensile strengthof 180 MPa, whereas sample No. II-7 having a thick network portion had acoefficient of elasticity of 250 GPa and tensile strength of 60 MPa. Itis noted that a coefficient of elasticity and tensile strength weremeasured with a commercially available measurement apparatus.

Moreover, as shown in Table 3, it can be seen that, in a case where anSiC aggregate containing Si is fabricated and sintered in a nitrogenatmosphere, the network portion tends to be thick even though thesintering temperature is lower than 2000° C.

The composite member satisfying a coefficient of thermal expansion from4 ppm/K to 8 ppm/K in Test Example II-2 is not only excellent inadaptability to a semiconductor element or peripheral thereof having acoefficient of thermal expansion around 4 ppm/K but also high in thermalconductivity. Therefore, these composite members are also expected tosuitably be made use of as a constituent material for a heat radiationmember of the semiconductor element above.

Embodiment II-2

A composite member including a substrate composed of a compositematerial made of a composite of pure magnesium and SiC and a metalcoating layer covering each of two opposing surfaces of the substratewas fabricated, and thermal characteristics of the obtained compositemember were examined.

An ingot of pure magnesium as in Test Example II-1 in Embodiment II-1and an SiC sintered body were prepared as raw materials. In addition,the SiC sintered body was subjected to oxidation treatment as in TestExample II-1 in Embodiment II-1. Moreover, a pair of plate-shapedspacers each having a size of 10 mm long×100 mm wide×0.5 mm thick andmade of carbon was prepared.

Here, a cast having such a size as allowing arrangement of the spacerabove between the SiC sintered body and the cast is made use of. Such astate that the SiC sintered body and the pair of spacers wereaccommodated in the cast to which a release agent had been applied asappropriate and the pair of spacers sandwiched the SiC sintered body wasset. Being sandwiched between the spacers above, the sintered body isarranged in the cast in a stable manner, and a gap corresponding to athickness of the spacer (a gap of 0.5 mm here) is provided between theSiC sintered body and the cast. This cast was loaded into an atmospherefurnace as in Embodiment II-1. Then, a composite of the SiC sinteredbody and molten pure magnesium was made under the conditions the same asin Embodiment II-1. In this composite step, molten pure magnesium flowsinto a gap between the cast and the SiC sintered body provided by thespacer, so that a metal coating layer made of pure magnesium was formedon each of two opposing surfaces of the substrate made of a composite.

As described above, the composite member including the substratecomposed of the composite material made of a composite of SiC and puremagnesium and the metal coating layer composed of pure magnesium on eachof the two opposing surfaces of the substrate was obtained. Thecross-section of the obtained composite member was observed with anoptical microscope (×50 magnification). Then, it could be confirmed thatSiC was in a web shape, that is, a network portion was formed, as shownin FIG. 5. In addition, it could be confirmed that the gap between SiCand SiC was infiltrated with pure magnesium and the metal coating layercomposed of pure magnesium was provided on the surface of the substrateabove. Composition of a constituent metal of this substrate and themetal coating layer was examined with an EDX apparatus, and it was foundthat the composition was the same (pure magnesium). Moreover, it couldbe confirmed from the observed image of the cross-section above thateach metal coating layer had a texture continuous to pure magnesium inthe substrate above. Further, the observed image of the cross-sectionabove was used to measure a thickness of each metal coating layer, andit was approximately 0.5 mm (500 μm). It could be confirmed that thisthickness was substantially identical to the thickness of the spacerabove.

An SiC content in a portion where a composite of pure magnesium and SiCwas made, that is, a portion other than the metal coating layer, in theobtained composite member, was measured, and it was 80 volume %. The SiCcontent was measured as in Embodiment II-1.

Moreover, coefficient of thermal expansion α (ppm/K) and thermalconductivity κ (W/m·K) of the obtained composite member were measured.Then, coefficient of thermal expansion α was 5.1 ppm/K and thermalconductivity κ was 250 W/m·K. The coefficient of thermal expansion andthe thermal conductivity were measured as in Embodiment II-1.

From the foregoing, the obtained composite member is not only excellentin adaptability to a semiconductor element or peripherals thereof havinga coefficient of thermal expansion around 4 ppm/K but also high inthermal conductivity. Therefore, the composite member is expected tosuitably be made use of as a constituent material for a heat radiationmember of the semiconductor element above. In addition, the compositemember according to Embodiment II-2 can be plated with Ni throughelectroplating, as it includes the metal coating layers on respectiveopposing surfaces of a base member. Plating with Ni enhancessolderability. Even in a case where the composite member is made use ofin a semiconductor device for which solder is desired, the compositemember can sufficiently adapt thereto. Further, in the composite memberaccording to Embodiment II-2, a thickness or a region to be formed ofthe metal coating layer can readily be varied by selecting a thicknessor a shape of a spacer as appropriate.

Though a construction in which metal coating layers are formed onrespective opposing surfaces of a substrate made of a composite materialhas been described in Embodiment II-2 above, a metal coating layer maybe formed only on any one surface.

In this case, a spacer is preferably arranged only on one surface of anSiC aggregate.

Third Form: Formation of Metal Coating Layer Test Example III-1

A composite member was fabricated with the use of a cast made of aspecific material. Here, an ingot of pure magnesium composed of 99.8mass % or more Mg and an impurity and particulate SiC powders (particlesize from 10 to 170 μm, average particle size: 120 μm) were prepared asraw materials. All raw materials used were commercially available.

A cast including a main body portion 11 and a lid portion 12 that areintegrated by a screw 13 as shown in FIG. 7 was made use of as the cast.It is noted that FIG. 7(A) shows a lateral dimension of the main bodyportion in an exaggerated manner for the sake of illustration of aninternal structure of the cast. A cast 10 is in a hollow prism shapewith bottom and opens on one surface side, with lid portion 12 in arectangular plate shape being fixed to main body portion 11. The insideof main body portion 11 has a stepped space. A space small in depth froman opening portion serves as a metal placement portion, while a spacelarge in depth from the opening portion serves as a composite memberformation portion. An ingot M (see FIG. 8) serving as a metal componentof the substrate is arranged on one surface (a metal placement surface11 m) in parallel to a bottom surface 11 b in the metal placementportion. The composite member formation portion is a space surrounded byone surface in parallel to bottom surface 11 b (an SiC placement surface11 s), a coupling surface 11 c coupling both surfaces 11 m and 11 s toeach other, and an inner surface 12 i of 11 d portion 12 (thickness t(t_(s)): 4.5 mm, width w: 100 mm, length 1 (magnitude in a direction ofdepth): 200 mm) This composite member formation space is filled with SiCpowders, to thereby form an SiC aggregate S (see FIG. 8). Main bodyportion 11 and lid portion 12 are made of carbon (α_(m)=4 (ppm/K)).Screw 13 is made of SUS304 (coefficient of thermal expansion α_(N): 17.3ppm/K), and 10 mm of its length: 15 mm is buried in the cast (t_(N)=10(mm))

Lid portion 12 was fixed to main body portion 11 by screw 13 to assemblecast 10, and the composite member formation space was filled with SiCpowders by tapping, to thereby fabricate the SiC aggregate (α_(S)=3(ppm/K)). Then, ingot M of pure magnesium was arranged on metalplacement surface 11 m, and cast 10 was heated to a temperature notlower than a melting point of the metal above (875° C. here), so as tomelt the metal above (pure magnesium). Melting was carried out in an Aratmosphere at an atmospheric pressure. As a result of this heating,screw 13 higher in coefficient of thermal expansion than SiC expandedmore than main body portion 11 and lid portion 12, so that a small gapwas produced between the SiC aggregate and cast 10 (inner surface 12 iof lid portion 12) and molten metal (pure magnesium) flowed into thisgap. After the heated state above was held for 2 hours so as to make acomposite of the SiC aggregate and molten pure magnesium above, coolingwas performed in the Ar atmosphere (water cooling here).

In the micrograph in FIG. 6, a region of dark color on the lower siderepresents a background, and a region of light color on the upper siderepresents the composite member. In the composite member, a particulatesubstance represents SiC, a region where SiC particles are presentrepresents the substrate, and a region where SiC particles are notpresent represents the metal coating layer. Through the step above, asshown in FIG. 6, a composite member including a metal coating layercomposed of pure magnesium on one surface of the substrate containingpure magnesium as a base material metal, in which SiC particles aredispersed in this base material metal (SiC content: 65 volume %), wasobtained. An SiC content was found in such a manner that anycross-section of the composite member was observed with an opticalmicroscope (×50 magnification), the observed image was subjected toimage processing with a commercially available image analysis apparatus,a total area of SiC in this cross-section was found, a value obtained byconverting this total area into a volume ratio was adopted as a volumeratio based on this cross-section, a volume ratio in the cross-sectionfor n=3 was found, and an average value thereof was calculated.

The surface of the obtained composite member was examined. Then, onesurface side including a metal coating layer had less irregularitiesthan the other surface side not including a metal coating layer and thesurface was smooth. In addition, it can be seen that the base materialmetal of the substrate and the metal forming the metal coating layer inthe obtained composite member have a continuous texture. Composition ofa constituent metal of the base material metal and the metal coatinglayer above was examined with an EDX apparatus, and it was found thatthe composition was the same (pure magnesium). Moreover, a thickness ofthe metal coating layer was examined based on a photograph of thecross-section. Then, it was approximately 90 μm on average and the metalcoating layer was formed uniformly on one surface of the substrate. Thisthickness of the metal coating layer was substantially the same as theresult calculated in Equation (1) described above. Further, thermalconductivity and a coefficient of thermal expansion of the obtainedcomposite member were measured with a commercially available measuringinstrument, and they were 208 W/m·K and 8 ppm/K, respectively. Thermalconductivity and a coefficient of thermal expansion of a portioncomposed of a composite material were similarly measured, with theformed metal coating layer being removed, and then they were 210 W/m·Kand 7.8 ppm/K, respectively.

The composite member above was plated with Ni through electroplating.Then, uniform Ni plating could be formed on the metal coating layer. Inaddition, corrosion resistance and solderability of a composite memberincluding Ni plating and a composite member not plated with Ni wereexamined. Then, the composite member plated with Ni was superior incorrosion resistance and solderability.

A heating temperature of the cast is preferably set to a temperature atwhich the ingot is molten but it is not boiled, specifically not lowerthan 650° C. and not higher than 1000° C. In addition, pores may bereduced by subjecting the obtained composite member or the substrate tohot pressing (heating temperature: not lower than 300° C. and preferablynot lower than 600° C., applied pressure: not lower than 1 ton/cm²) orthe like. Moreover, wettability between SiC and the molten metal (puremagnesium here) may be enhanced by forming an SiC aggregate with the useof coated SiC obtained by heating raw material SiC to 700° C. or higherto form an oxide layer satisfying a mass ratio to raw material SiC notlower than 0.4% and not higher than 1.5%. The heating temperature of thecast, hot pressing of the composite member or the substrate, and use ofcoated SiC described here are also applicable to Test Examples thatfollow.

Though SiC powders were made use of in the tests above, an SiC moldedbody may be made use of as in Test Examples which will be describedlater. Moreover, though an example where a metal coating layer isprovided on one surface of the substrate has been described in the testsabove, a construction may be such that metal coating layers are providedon two respective opposing surfaces of the substrate. In this case,preferably, the main body portion of the cast is further divided intotwo pieces to fabricate a divided piece including a coupling plate and adivided piece including an SiC mount surface, and the lid portion andboth divided pieces are integrated by means of the screw.

Test Example III-2

A composite member was fabricated with the use of an SiC molded body. Inthis test, molded bodies (I) to (III) below were prepared.

(I) Molded Body Made with Slip Casting

Slurry was fabricated by preparing particulate SiC powders used in TestExample III-1 and additionally a surfactant and water, setting a volumeratio between water and the SiC powders to water: SiC powders≈5:5, andthen adding the surfactant. Here, slurry of 20 mass % urea aqueoussolution (with the entire slurry being assumed as 100 mass %) and slurryof commercially available polycarboxylic-acid-based aqueous solutionwere prepared. Each slurry was poured into the cast followed by airdrying, to thereby fabricate a powder molded body.

(II) Molded Body Made with Pressure Forming

A powder molded body was obtained by preparing particulate SiC powdersused in Test Example III-1, adding and mixing ammonium chloride asappropriate as a binder in a range from 1 to 10 mass %, filling the castwith this mixture, and applying pressure of 3 ton/cm². It is noted thatthe binder was emanated through decomposition and vaporization by heatgenerated during fabrication of the substrate.

(III) Sintered Molded Body (Sintered Body)

A sintered body A obtained by sintering the powder molded bodyfabricated in (I), (II) above in atmosphere at 1000° C.×2 hours and asintered body B obtained by sintering the same in vacuum at 2000° C.×8hours were prepared. In addition, commercially available SiC sinteredbodies α and β were prepared. Sintered body B sintered in vacuum andcommercially available SiC sintered bodies α and β were observed with anSEM. Then, presence of a network portion where SiC was directly bondedto each other was observed. As a result of similar observation ofsintered body A sintered in atmosphere, presence of a network portionwhere SiC was bonded with an oxide layer being interposed was observed.

Each prepared molded body above was slightly smaller in thickness thanthe composite member formation space in the cast having thickness t (5mm here). In addition, a cast having a shape the same as shown in FIGS.7 and 8 and including a screw made of carbon was prepared as the cast.Then, in this test, such a sample that each molded body alone wasaccommodated in the composite member formation space in this cast andsuch a sample that each molded body and a spacer were accommodated wereprepared. In the sample that each molded body was accommodated in thecomposite member formation space in the cast above, a small gap isproduced between the molded body and the cast (the inner surface of thelid portion). Meanwhile, a carbon sheet, a naphthalene plate, and a wirewere prepared as the spacer. Carbon sheets and naphthalene plates havingthicknesses of 50 μm, 100 μm, 200 μm, 500 μm, 1000 μm, and 1500 μm wereprepared. Then, a pair of carbon sheets identical in thickness or a pairof naphthalene plates identical in thickness was arranged between themolded body and the cast so as to sandwich two opposing surfaces of themolded body, so that a gap corresponding to a thickness of each spacerwas produced between one surface of the molded body above and the castand between the other surface thereof and the cast. A wire having adiameter of 0.05 mm (50 μm) and made of SUS430 was prepared as the wire.The molded body was arranged in a central portion in a direction ofthickness in the cast, and the wire above fixed the molded body to thecast such that a gap having a prescribed size is evenly provided betweenone surface of the two opposing surfaces of the molded body and the castand between the other surface thereof and the cast (a size of a gapbetween one surface of the molded body and the cast: 50 μm, 100 μm, 200μm, 500 μm, 1000 μm, and 1500 μm).

While the molded body or the molded body and the spacer was (were)accommodated in the cast as described above, the ingot of pure magnesiumas in Test Example III-1 was arranged on the metal placement surface ofthe cast, the ingot above was molten under the conditions the same as inTest Example III-1 (Ar atmosphere, atmospheric pressure, 875° C.×2hours), the molded body (SiC aggregate) was infiltrated with a moltenmetal (pure magnesium) for making a composite, and the molten metalabove was poured into the gap above followed by cooling. It is notedthat the naphthalene plate disappeared as a result of sublimation duringheating of the cast. Alternatively, in a case where a spacer such as acarbon sheet is made use of, in order to prevent displacement of aposition of arrangement of the spacer with respect to the molded body,the spacer may adhere to the molded body with the use of low-meltingglass, low-melting salt, water glass, or the like.

Through the step above, the composite member (thickness of 5 mm)including the metal coating layers made of pure magnesium on therespective opposing surfaces of the substrate made of a composite ofpure magnesium and SiC was obtained. In particular, the sample obtainedby making use of the powder molded body was a substrate in which SiC wasdispersed in pure magnesium.

Each obtained sample was observed as in Test Example III-1. Then, themetal coating layers were formed uniformly on the respective opposingsurfaces of the substrate in any sample. In the sample in which thecarbon sheet was made use of, the carbon sheet was visually recognizedas remaining on the surface of the metal coating layer. In the sample inwhich the naphthalene plate was made use of, no foreign matter wasrecognized on the surface of the metal coating layer. In the sample inwhich the wire was made use of, a part of the wire was visuallyrecognized in a part of the metal coating layer. Specifically, in thesample in which a gap provided between one surface of the molded bodyand the cast is substantially equal to the diameter of the wire (gap: 50μm), a part of the wire remained on a pair of main surfaces eachincluding the metal coating layer in the composite member and on a sidesurface orthogonal to the main surface. In the sample in which the gapabove is greater than the diameter of the wire (gap: 100 μm, 200 μm, 500μm, 1000 μm, and 1500 μm), though a part of the wire remained on a sidesurface of the composite member, it was recognized on neither of themain surfaces of the composite member and the wire was buried in themetal coating layer. A thickness of each metal coating layer wasexamined as in Test Example III-1. Then, the thickness in the sample notincluding the spacer was 200 μm on average (the sum of thicknesses ofboth metal coating layers: 0.4 mm on average). In any sample includingthe spacer, thicknesses of each metal coating layer provided on the mainsurface were 50 μm, 100 μm, 200 μm, 500 μm, 1000 μm, and 1500 μm onaverage (the sum of thicknesses of both metal coating layers: 0.1 mm,0.2 mm, 0.4 mm, 1 mm, 2 mm, and 3 mm on average), and the thickness wassubstantially the same as the size of the gap provided between onesurface of the molded body and the cast. In addition, a texture of thebase material metal and the constituent metal of the metal coating layerwas examined as in Test Example III-1, and it was found as a continuoustexture. By making use of a spacer or the like, as compared with a casewhere a molded body smaller than a cast is merely made use of, a gap canreliably be provided between a molding plate and the cast, and a metalcoating layer having a uniform thickness can be formed. Further, bychanging arrangement of the spacer above or a state of fixing of thewire as appropriate, a composite member including a metal coating layeronly on one surface of the substrate or on each of two opposing surfacesthereof can be formed in a simplified manner.

An SiC content in the substrate in each sample fabricated with themolded body in (I), (II), sintered bodies obtained by sintering thesemolded bodies, and a commercially available sintered body, thermalconductivity and a coefficient of thermal expansion of each sample, aswell as thermal conductivity and a coefficient of thermal expansion ofthe substrate alone from which the metal coating layer had been removedwere found as in Test Example III-1. Table 4 shows the results. As shownin Table 4, it can be seen that a composite member low in coefficient ofthermal expansion and high in thermal conductivity is obtained when thesum of thicknesses of both metal coating layers is not greater than 2.5mm and further not greater than 0.5 mm. In addition, it can be seenthat, in a case of making use of a sintered body as the molded body, inparticular, making use of a sintered body having a network portion whereSiC is directly bonded to each other, a composite member low incoefficient of thermal expansion and high in thermal conductivity isobtained in spite of the SiC content being the same.

TABLE 4 SiC Amount Thermal Conductivity (W/m · K) in Base Thickness ofMetal Coating Layer (μm) Sintering Material 0 (No Molded Body Condition(Volume %) Coating) 50 100 200 500 1000 1500 (I) No Sintering 70% 220218 217 210 205 195 181 Slip Casting Atmosphere 70% 215 204 203 201 196188 180 1000° C. Vacuum 70% 247 245 242 238 225 207 192 2000° C. (II) NoSintering 75% 224 222 221 218 210 197 186 Pressure Forming Atmosphere75% 219 208 207 205 199 190 181 1000° C. Vacuum 75% 258 255 252 247 233212 194 2000° C. Commercially — 80% 263 260 257 251 236 214 196Available Sintered Body α Commercially — 84% 282 278 274 267 248 221 199Available Sintered Body β Test Example — 65% 212 211 210 208 201 191 182III-3 Tapping Coefficient of Thermal Expansion (ppm/K) Thickness ofMetal Coating Layer (μm) 0 (No Molded Body Coating) 50 100 200 500 10001500 (I) 7.5 7.5 7.6 7.9 8.7 10.3 12.8 Slip Casting 7.4 7.5 7.6 7.8 8.510.1 12.5 5.1 5.2 5.3 5.4 6 7.4 9.6 (II) 7.2 7.3 7.4 7.6 8.3 9.8 12.1Pressure Forming 7.1 7.2 7.3 7.5 8.2 9.7 12 4.6 4.7 4.8 4.9 5.5 6.8 8.9Commercially 4.4 4.5 4.5 4.7 5.2 6.4 8.5 Available Sintered Body αCommercially 4.1 4.2 4.2 4.4 4.9 6 8 Available Sintered Body β TestExample 7.8 7.9 8 8.2 9 10.6 13 III-3 Tapping

Test Example III-3

An SiC aggregate was formed by tapping SiC powders, and a compositemember was fabricated by making use of a spacer that can be removed byheating.

A plate made of naphthalene (thickness t_(n): 50 μm, 100 μm, 200 μm, 500μm (0.5 mm), 1000 μm, and 1500 μm, width w_(n): 100 mm, length l_(n):200 mm) was prepared as a spacer. In addition, particulate SiC powdersused in Test Example III-1 and an ingot of pure magnesium serving as ametal component of the substrate as well as a cast used in Test ExampleIII-2 (thickness t: 5 mm) were prepared. The naphthalene plate wasarranged in contact with the inner surface of the lid portion of thecast and the coupling surface and the cast was filled with the SiCpowders by tapping in this state, so as to form the SiC aggregate in thecomposite member formation space of the cast, and such a state that thenaphthalene plate was in contact with opposing surfaces of the SiCaggregate was set. Then, the ingot of pure magnesium was arranged on themetal placement surface and the cast was heated to a temperature notlower than a melting point of pure magnesium (approximately 650° C.).When the temperature attained to a temperature of sublimation ofnaphthalene during heating, naphthalene sublimed and it was removed fromthe composite member formation space. Thus, a gap was produced in aspace where the naphthalene plate had been present. Namely, a gap wasproduced between the SiC aggregate and the cast. Heating was continuedfurther to a prescribed temperature (875° C.) in this state so as tomelt the ingot above under the conditions the same as in Test ExampleIII-1 (Ar atmosphere, atmospheric pressure, 875° C.×2 hours), the SiCaggregate was infiltrated with the molten metal (pure magnesium) formaking a composite, and the molten metal above was poured into the gapabove followed by cooling.

Through the step above, as in Test Example III-1, the composite member(thickness: 5 mm) including the metal coating layers made of puremagnesium on respective opposing surfaces of the substrate containingpure magnesium as a base material metal, in which SiC particles weredispersed in this base material metal (SiC content: 65 volume %), wasobtained. In addition, each metal coating layer above had a thickness of50 μm, 100 μm, 200 μm, 500 μm (0.5 mm), 1000 μm, and 1500 μm on average(the sum of thicknesses of both metal coating layers: 0.1 mm, 0.2 mm,0.4 mm, 1 mm, 2 mm, and 3 mm on average), and the thickness wassubstantially the same as the thickness of the used naphthalene plate.Confirmation of a state of the composite member, measurement of athickness of the metal coating layer, and measurement of the SiC contentin the substrate were carried out as in Test Example III-1. In addition,thermal conductivity and a coefficient of thermal expansion of eachsample and thermal conductivity and a coefficient of thermal expansionof the substrate alone from which the metal coating layer had beenremoved were found as in Test Example III-1. Table 4 shows the results.

Test Example III-4

A metal plate was arranged in the cast to fabricate a composite member.

An Al plate (a plate made of pure aluminum of JIS alloy number 1050,thickness t_(a): 0.5 mm, width w_(a): 100 mm, length l_(a): 200 mm) wasprepared as the metal plate. In addition, particulate SiC powders usedin Test Example III-1 and an ingot of pure magnesium serving as a metalcomponent of the substrate as well as a cast used in Test Example III-2were prepared. The Al plate was arranged in contact with the innersurface of the lid portion of the cast or the coupling surface and thecast was filled with the SiC powders by tapping in this state, so as toform the SiC aggregate in the composite member formation space of thecast, and such a state that the Al plate was brought in contact with onesurface of the SiC aggregate was established. Then, the ingot of puremagnesium was arranged on the metal placement surface of the cast, theingot above was molten under the conditions the same as in Test ExampleIII-1 (Ar atmosphere, atmospheric pressure, 875° C.×2 hours), the SiCaggregate was infiltrated with the molten metal (pure magnesium) formaking a composite, and the Al plate was joined to the SiC aggregatewith the use of the molten metal above followed by cooling.

Through the step above, the composite member including the metal coatinglayer made of a metal different in composition from the base materialmetal (pure aluminum here) on one surface of the substrate containingpure magnesium as a base material metal, in which SiC particles weredispersed in this base material metal (SiC content: 65 volume %), wasobtained. In addition, the metal coating layer above had a thickness of0.5 mm on average, and the thickness was substantially the same as thethickness of the used Al plate. Confirmation of a state of the compositemember, measurement of a thickness of the metal coating layer, andmeasurement of the SiC content in the substrate were carried out as inTest Example III-1.

Test Example III-5

A metal plate was joined to a substrate made of a composite materialwith a hot pressing method, to fabricate a composite member.

In this test, an ingot of pure magnesium serving as the metal componentof the substrate used in Test Example III-1 and a cast used in TestExample III-2 (it is noted that thickness t: 4 mm) as well as acommercially available SiC sintered body (thickness: 4.0 mm, width: 100mm, length: 200 mm) were prepared. Then, the substrate made of thecomposite material was fabricated by arranging the SiC sintered body(SiC aggregate) above in the composite member formation space of thecast, arranging the ingot of pure magnesium on the metal placementsurface of the cast, melting the ingot above under the conditions thesame as in Test Example III-1 (Ar atmosphere, atmospheric pressure, 875°C.×2 hours), and infiltrating the SiC sintered body with the moltenmetal (pure magnesium) for making a composite. Through the step above,the substrate made of a composite of pure magnesium and SiC was obtained(SiC content: 84 volume %, thickness: 4.0 mm, width: 100 mm, length: 200mm) The SiC content in the substrate was found as in Test Example III-1.

In addition, in this test, two Mg plates (MIS1), two Al plates (JISalloy number: 1050), two Cu plates (JIS number: C1020), and two Niplates (NAS Ni201), each of which is made of pure metal of which purityis not lower than 99% and has a thickness of 0.5 mm, a width of 100 mmand a length of 200 mm, were prepared as metal plates. Any metal plateis commercially available. A stack was prepared by sandwiching afabricated substrate between a pair of metal plates identical incomposition, this stack was arranged in a box-shaped pressure mold thatcan be heated, and this stack was heated by heating the pressure moldand then a punch was pressed against a surface of one metal plateexposed through an opening portion of the pressure mold above. Table 5shows a heating temperature of the stack above, an applied pressure, anda joint atmosphere during heating and pressurization. In addition, astate of joint of the stack after heating and pressurization above wasexamined Table 5 shows the results. Regarding the joint state above, thestack after heating and pressurization was cut in a direction ofthickness, and such a state that the entire surface was joined withoutany gap between the substrate and the metal plate in the cut surface wasevaluated as “∘” and such a state that the metal plate was detached fromthe substrate and not joined was evaluated as “x”.

TABLE 5 Heating Sample Temperature Applied Pressure Metal PlateComposition No. ° C. ton/cm² Joint Atmosphere Mg Al Cu Ni III-1 300 0.5Atmosphere x x x x III-2 300 1 Atmosphere x x x x III-3 300 3 Atmospherex x x x III-4 300 5 Atmosphere x x x x III-5 300 0.5 Ar x x x x III-6300 1 Ar x x x x III-7 300 3 Ar ∘ ∘ x x III-8 300 5 Ar ∘ ∘ x x III-9 3500.5 Atmosphere x x x x III-10 350 1 Atmosphere x x x x III-11 350 3Atmosphere x x x x III-12 350 5 Atmosphere x x x x III-13 350 0.5 Ar x xx x III-14 350 1 Ar x x x x III-15 350 3 Ar ∘ ∘ x x III-16 350 5 Ar ∘ ∘x x III-17 400 0.5 Atmosphere x x x x III-18 400 1 Atmosphere x x x xIII-19 400 3 Atmosphere x x x x III-20 400 5 Atmosphere ∘ ∘ x x III-21400 0.5 Ar x x x x III-22 400 1 Ar x x x x III-23 400 3 Ar ∘ ∘ x xIII-24 400 5 Ar ∘ ∘ x x III-25 500 0.5 Atmosphere ∘ ∘ x x III-26 500 1Atmosphere ∘ ∘ x x III-27 500 3 Atmosphere ∘ ∘ x x III-28 500 5Atmosphere ∘ ∘ x x III-29 500 0.5 Ar ∘ ∘ ∘ ∘ III-30 500 1 Ar ∘ ∘ ∘ ∘III-31 500 3 Ar ∘ ∘ ∘ ∘ III-32 500 5 Ar ∘ ∘ ∘ ∘ III-33 600 0.5Atmosphere ∘ ∘ x x III-34 600 1 Atmosphere ∘ ∘ x x III-35 600 3Atmosphere ∘ ∘ ∘ ∘ III-36 600 5 Atmosphere ∘ ∘ ∘ ∘ III-37 600 0.5 Ar ∘ ∘∘ ∘ III-38 600 1 Ar ∘ ∘ ∘ ∘ III-39 600 3 Ar ∘ ∘ ∘ ∘ III-40 600 5 Ar ∘ ∘∘ ∘ III-41 645 0.5 Atmosphere ∘ ∘ ∘ ∘ III-42 645 1 Atmosphere ∘ ∘ ∘ ∘III-43 645 3 Atmosphere ∘ ∘ ∘ ∘ III-44 645 5 Atmosphere ∘ ∘ ∘ ∘ III-45645 0.5 Ar ∘ ∘ ∘ ∘ III-46 645 1 Ar ∘ ∘ ∘ ∘ III-47 645 3 Ar ∘ ∘ ∘ ∘III-48 645 5 Ar ∘ ∘ ∘ ∘

As shown in Table 5, it can be seen that metal plates made not only of ametal identical in type to the metal component of the substrate but alsoof a metal different in type can be joined to the substrate made of acomposite material by performing heating and pressurization. Inparticular, it can be seen that a composite member including a metalcoating layer is obtained by heating to a temperature not lower than300° C. in a case where a metal plate is made of Mg, Al and an alloythereof and by heating to a temperature not lower than 600° C. in a casewhere a metal plate is made of Cu, Ni and an alloy thereof. Moreover, itcan be seen that the applied pressure is preferably not lower than 0.5ton/cm². Further, in the sample where the substrate and the metal platewere joined to each other, the metal plate was firmly joined to thesubstrate as it plastically deformed. Additionally, it can be concludedfrom this test that, if heating and pressurization conditions are thesame, a joint state varies depending on a joint atmosphere, and an Ar(argon) joint atmosphere facilitates joint as compared with anatmosphere. A thickness of each metal coating layer formed on each ofthe opposing surfaces of the substrate was measured as in Test ExampleIII-1, and it was 0.5 mm on average (the sum of thicknesses of bothmetal coating layers: 1 mm, a thickness of the obtained compositemember: 5 mm), which was substantially the same as the thickness of theused metal plate.

Furthermore, thermal conductivity and a coefficient of thermal expansionof the sample in which the metal coating layers were formed onrespective opposing surfaces of the substrate were found as in TestExample III-1. Table 6 shows the results. As shown in Table 6, it can beseen that a composite member further higher in thermal conductivity isobtained by changing as appropriate composition of the metal coatinglayer.

TABLE 6 Metal Coating Layer Thermal Conductivity Coefficient of ThermalComposition (W/m · K) Expansion (ppm/K) Mg 248 4.9 Al 272 5.2 Cu 299 5.4Ni 234 5.6

Fourth Form: Large Composite Member Embodiment IV-1

A composite member (substrate) made of a composite of pure magnesium andSiC was fabricated under various conditions, and a state of defect andthermal characteristics of the obtained composite member were examined.

An ingot of pure magnesium composed of 99.8 mass % or more Mg and animpurity and a commercially available SiC sintered body (relativedensity 80%, 200 mm long (long side)×100 mm wide (short side)×5 mmthick) were prepared as raw materials.

The prepared SiC sintered body was subjected to oxidation treatment at875° C.×2 hours so as to form an oxide film, so that wettability withmolten pure magnesium was enhanced. The step of oxidation treatmentabove may not be performed.

The SiC sintered body above was accommodated in cast 10 shown in FIG. 9and a sintered body 20 was infiltrated with molten pure magnesium, tofabricate an infiltrated plate.

Cast 10 above is a box in a vertically long parallelepiped shape openingon one side, and it includes main body portion 11 and lid portion 12 asshown in FIG. 9. Main body portion 11 includes a rectangular bottomsurface portion 10 b, a pair of rectangular end surface wall portions 10e erecting from bottom surface portion 10 b, and a sidewall portion 10 serecting from bottom surface portion 10 b and coupled to the pair of endsurface wall portions 10 e. Sidewall portion 10 s is in a verticallylong rectangular shape as shown in FIG. 9. Lid portion 12 is arectangular plate body equal in vertical length to sidewall portion 10s, and it is arranged opposed to sidewall portion 10 s and fixed tobottom surface portion 10 b and the pair of end surface wall portions 10e with a bolt (not shown). As lid portion 12 is attached to main bodyportion 11, vertically long cast 10 opening on a side opposed to bottomsurface portion 10 b is constructed. Here, cast 10 is made of carbon. Aninternal space of this cast 10 is made use of as a space foraccommodating sintered body 20. Here, the internal space of cast 10 hasa size corresponding to the sintered body above, such that substantiallyno gap is provided between sintered body 20 and cast 10 when sinteredbody 20 is accommodated in cast 10. Lid portion 12 may be attached aftersintered body 20 is arranged in main body portion 11, or sintered body20 may be arranged after cast 10 is assembled. An integrally molded castmay be made use of, instead of a construction in which divided piecesare combined.

In addition, here, sintered body 20 was accommodated in cast 10 after acommercially available release agent had been applied to a portion ofcontact of an inner surface of cast 10 with the sintered body. Releaseof the composite member can be facilitated by applying the releaseagent. The step of applying this release agent may not be performed.

Cast 10 above has an ingot placement portion (not shown) coupled to aperiphery of an opening portion, on which the prepared ingot above isarranged. By heating this cast 10 to a prescribed temperature, the ingotis molten. Heating of cast 10 is carried out by making use of anatmosphere furnace 30 shown in FIG. 11. Atmosphere furnace 30 includes avessel 31 that can hermetically be sealed, a heat-insulating material 32arranged on an inner surface of vessel 31, and a heater 33 arranged in aspace surrounded by heat-insulating material 32. Vessel 31 includes abottom surface portion, a sidewall portion erecting from the bottomsurface portion, and an upper surface portion arranged opposed to thebottom surface portion and coupled to the sidewall portion.Heat-insulating material 32 is arranged along the bottom surfaceportion, the sidewall portion and the upper surface portion of vessel 31above. Here, atmosphere furnace 30 is arranged such that the bottomsurface portion thereof is located on a lower side in a verticaldirection and the upper surface portion is located on an upper side inthe vertical direction. In addition, here, cast 10 is loaded such thatbottom surface portion 10 b of cast 10 is in contact with the bottomsurface portion of atmosphere furnace 30 above. Namely, cast 10 isloaded into atmosphere furnace 30 such that bottom surface portion 10 bof cast 10 is located on the lower side in the vertical direction andthe opening portion of cast 10 is located on the upper side in thevertical direction.

Here, atmosphere furnace 30 above was adjusted such that theinfiltration temperature was set to 775° C., an Ar atmosphere was set,and a pressure in the atmosphere was set to an atmospheric pressure.Pure magnesium molten as a result of heating by heater 33 flowed intothe internal space of cast 10 through the opening portion of cast 10 byits own weight and sintered body 20 arranged in the internal space isinfiltrated therewith. The infiltrated plate is thus obtained.

Cast 10 in which the infiltrated plate was arranged was cooled with thefollowing cooling methods (1) to (7) to solidify pure magnesium. Thus, acomposite member having a size of 200 mm long×100 mm wide×5 mm thick wasobtained. In addition, a temperature gradient or a cooling rate whichwill be described later was varied by adjusting as appropriate athickness of a heat-insulating material, an amount of air sent by a fan,a temperature of water-cooled copper, or the like in the followingcooling methods (2) to (7).

(1) Power of heater 33 of atmosphere furnace 30 was turned off togradually cool cast 10 within furnace 30. Namely, cast 10 is cooled in astate shown in FIG. 11. With this cooling method, the infiltrated platewithin the cast is cooled in a converging manner from its peripherytoward the central portion.

(2) After power of heater 33 of atmosphere furnace 30 was turned off,heat-insulating material 32 at the bottom surface portion within furnace30 was removed such that bottom surface portion 10 b of cast 10 comes indirect contact with vessel 31 of furnace 30. Cast 10 is thus cooledwithin furnace 30.

(3) After power of heater 33 of atmosphere furnace 30 was turned off,heat-insulating material 32 at the bottom surface portion within furnace30 was removed, a fan (not shown) was arranged outside furnace 30 in thevicinity of the bottom surface portion of furnace 30, and cast 10 withinfurnace 30 was cooled while air was sent from the fan toward the bottomsurface portion of furnace 30. Namely, cast 10 was forcibly cooled byair cooling.

(4) After power of heater 33 of the atmosphere furnace was turned off,heat-insulating material 32 at the bottom surface portion within thefurnace was partially removed and water-cooled copper 40 was attachedsuch that water-cooled copper 40 was brought in contact with bottomsurface portion 10 b of cast 10 as shown in FIG. 12(I). Cast 10 is thuscooled in the furnace. Namely, cast 10 is forcibly cooled by liquidcooling. It is noted that FIG. 12 does not show the vessel. Water-cooledcopper 40 is commercially available.

(5) After power of heater 33 of the atmosphere furnace was turned off,the end surface wall portion, sidewall portion 10 s, lid portion 12, andan outer peripheral surface of the opening portion of cast 10 arecovered with an heat-insulating material 34 as shown in FIG. 12(II).Namely, a portion other than the bottom surface portion of cast 10 iscovered with heat-insulating material 34. In this state, cast 10 iscooled as in (1) to (4) above. FIG. 12(II) shows a state that cast 10 iscovered with heat-insulating material 34 and bottom surface portion 10 bof cast 10 is brought in contact with water-cooled copper 40.

(6) After power of heater 33 of the atmosphere furnace was turned off,cast 10 is taken out of the furnace, and the bottom surface portion ofcast 10 is brought in contact with water-cooled copper 40, to therebycool cast 10. By taking out cast 10 from the atmosphere furnace andmaking use of water-cooled copper 40, the cooling rate can be increased.

(7) As shown in FIG. 13, an atmosphere furnace including in theatmosphere furnace (not shown), a high-temperature region includingheater 33 and an heat-insulating material 35 covering an outer peripherythereof and a low-temperature region provided under the high-temperatureregion by water-cooled copper 40 is prepared. Heater 33 andheat-insulating material 35 as well as water-cooled copper 40 arearranged to surround cast 10. Bottom surface portion 10 b of cast 10 issupported by a cylindrical support base 50 with bottom, and a movableportion 51 movable in a vertical direction by means of a drive portion(not shown) is arranged in support base 50. As a result of movement inthe vertical direction of this movable portion 51, support base 50 canmove in the vertical direction. Namely, cast 10 can move in the verticaldirection by means of movable portion 51. Cooling is performed by usingsuch an atmosphere furnace, driving movable portion 51 above, and movingcast 10 from the high-temperature region to the low-temperature region.Namely, cooling is performed by introduction from the bottom surfaceportion side of cast 10 arranged on the lower side in the verticaldirection into the low-temperature region.

In cooling the infiltrated plate, a temperature gradient and a coolingrate were measured as follows, with regard to the cooling methods (1) to(7) described above. Table 7 shows the results. Then, the compositemember was obtained by performing cooling in (1) to (7) described above,by using cast 10, atmosphere furnace 30 or the like achieving thetemperature gradient and the cooling rate shown in Table 7.

Thermocouples were arranged at regular intervals in the internal spaceof vertically long cast 10, along a longitudinal direction thereof fromthe side of bottom surface portion 10 b (the lower side in the verticaldirection) toward the opening portion side (the upper side in thevertical direction). Specifically, the thermocouples (not shown) areinstalled on sidewall portion 10 s (or lid portion 12) at 5 mm intervalfrom the surface of bottom surface portion 10 b on the inner surface ofcast 10. The cast is loaded into the atmosphere furnace in an emptystate where the inside of this cast 10 is not filled with pure magnesiumor SiC. The thermocouple is commercially available.

<Temperature Gradient>

A portion where the thermocouple above was arranged was defined as atemperature measurement point. When the temperature at each temperaturemeasurement point attained to 650° C., temperature difference:ΔT=T_(u)−T_(d) between two temperature measurement points P_(u) andP_(d) adjacent to this temperature measurement point P_(s), that is,between a temperature T_(u) at a temperature measurement point P_(u) onthe opening portion side (the upper side in the vertical direction) anda temperature T_(d) at a temperature measurement point P_(d) on thebottom surface portion side (the lower side in the vertical direction)with temperature measurement point P_(S) lying therebetween, iscalculated, and a value (T_(u)−T_(d))/1 obtained by diving thistemperature difference T_(u)−T_(d) by a distance 1 (10 mm here) betweentwo temperature measurement points P_(u) and P_(d) above is defined asthe temperature gradient. Table 7 shows a minimum value amongtemperature gradients calculated for 40 temperature measurement pointsprovided in cast 10. It is noted that the sample of which temperaturegradient value is “negative” in Table 7 is a sample in which coolingproceeds from the upper side in the vertical direction (the openingportion side of the cast) toward the lower side in the verticaldirection (the bottom surface portion side of the cast) in theinfiltrated plate accommodated in the cast, and here it corresponds tothe sample cooled in a converging manner from the periphery of theinfiltrated plate toward the central portion. The sample of whichtemperature gradient value is positive is a sample in which coolingproceeds from the lower side in the vertical direction (the bottomsurface portion side of the cast) toward the upper side in the verticaldirection (the opening portion side of the cast) in the infiltratedplate accommodated in the cast, and here it corresponds to the samplecooled in one direction from the lower side in the vertical direction inthe infiltrated plate toward the upper side in the vertical direction.

<Cooling Rate>

A portion where the thermocouple above was arranged was defined as atemperature measurement point. A time period t required for eachtemperature measurement point to decrease from a high temperature T_(H):680° C. to a low temperature T_(L): 620° C. was counted, and a value:(T_(H)−T_(L))/t obtained by dividing difference T_(H)−T_(L) betweentemperatures T_(H), T_(L) above by time period t above is defined as thecooling rate. Table 7 shows a minimum value among cooling ratescalculated for 40 temperature measurement points provided in cast 10.

An area ratio (%), difference in dimension (μm), surface roughness Ra(μm), thermal conductivity κ (W/m·K), and coefficient of thermalexpansion α (ppm/K) of a defect portion in the obtained composite memberwere measured. Table 7 shows the results.

The area ratio (%) of the defect portion was measured as follows. With astraight line passing through a center of gravity when the obtainedcomposite member is two-dimensionally viewed (here, an intersection ofdiagonals of a rectangular surface of 200 mm×100 mm) being defined as asection line, the composite member was cut using ion beam machining anda cross-section in a direction of thickness (a cross-section having across-sectional area of 200 mm×5 mm here) is taken. In thiscross-section, a region in a range extending over up to 10% of a length(200 mm here) of the section line above along the longitudinal directionof the section line above, with the center of gravity above serving asthe center, is defined as a central region. Any 20 small regions of 1mm×1 mm were selected in this central region (a region having across-sectional area: 40 mm×5 mm here) and a ratio of an area of thedefect portion with respect to the area of each small region: an arearatio is calculated. The area of the defect portion is found by makinguse of an image of the cross-section above. Specifically, the imageabove is subjected to image analysis with the use of a commerciallyavailable image processing apparatus, a portion in each small regionother than pure magnesium and SiC forming the composite member (mainly avoid) is determined as the defect portion in the small region, and thetotal area thereof is found with the image processing apparatus above.Then, Table 7 shows a maximum value of the area ratio of 20 defectportions in each sample.

Regarding the difference in dimension, largest thickness and smallestthickness were measured in the image of the cross-section above(cross-sectional area: 200 mm×5 mm) of each sample, and differencebetween these largest thickness and smallest thickness was calculated.If this difference is not smaller than 200 μm (0.2 mm), it can be saidthat shrinkage at the surface is great and surface property is poor. Ifthe difference is not greater than 50 μm (0.05 mm), it can be said thatshrinkage at the surface is very small.

Surface roughness Ra was measured in compliance with JIS B 0601 (2001).

Coefficient of thermal expansion α and thermal conductivity κ weremeasured by cutting a test piece from the obtained composite member andby using a commercially available measuring instrument. Coefficient ofthermal expansion α was measured in a range from 30° C. to 150° C.

In addition, regarding the sample of which area ratio of the defectportion is not higher than 10% among the obtained composite members, acomponent of the composite member, a shape of SiC, an SiC content, and acomposite state were examined Components of the composite member wereexamined with the use of an EDX apparatus. Then, the components were Mgand SiC and the remainder of inevitable impurities, that were the sameas in the used raw materials. In addition, the obtained composite memberwas subjected to a CP (Cross-section Polisher) process to expose across-section, that was examined in SEM observation. Then, SiC wasdirectly bonded to each other. Namely, such a porous body that a networkportion was formed of SiC was obtained, that was the same as in the usedraw material sintered body. In addition, the cross-section of the samplewas observed with an optical microscope (×50 magnification). Then, itcould be confirmed that a gap between SiC and SiC was infiltrated withpure magnesium as shown in FIG. 10(I). A portion forming a continuousweb shape in FIG. 10 represents SiC, while a portion clustered like aparticle represents pure magnesium.

Regarding the SiC content above, any cross-section of the compositemember was observed with an optical microscope (×50 magnification), theobserved image was subjected to image processing with a commerciallyavailable image analysis apparatus, a total area of SiC in thiscross-section was found, a value obtained by converting this total areainto a volume ratio was adopted as a volume ratio based on thiscross-section, a volume ratio in the cross-section for n=3 was found,and an average value thereof was calculated (area ratio≈volume ratio).Consequently, the SiC content was 80 volume %.

TABLE 7 Coefficient of Temperature Area Ratio of Difference in SurfaceThermal Thermal Sample Cooling Rate Gradient Defect Portion DimensionRoughness Conductivity Expansion No. (° C./min) (° C./mm) (%) (μm) (μm)(W/m · K) (ppm/K) IV-1 2.29 0.136 2.06 34 1.8 283 4.1 IV-2 2.29 0.1043.04 48 2.2 270 4.1 IV-3 2.29 0.056 4.41 175 2.6 253 3.9 IV-4 2.29 0.00210.4 763 7.5 164 4.2 IV-5 2.29 −0.001 13.4 982 12.1 136 4.1 IV-6 3.190.516 0.72 8 1.7 301 4 IV-7 3.19 0.284 1.36 16 2 292 4.1 IV-8 3.19 0.1761.69 17 2 288 4.1 IV-9 4.14 0.676 0.36 4 1.5 305 4.2 IV-10 4.14 0.3400.93 10 1.6 298 4.2 IV-11 4.14 0.252 1.34 12 1.9 293 4 IV-12 4.19 0.7640.25 3 1.4 307 4 IV-13 4.19 0.420 0.78 7 1.6 300 4.1 IV-14 4.19 0.2920.84 13 1.9 299 4.1 IV-15 3.16 0.044 4.9 428 3.4 244 4 IV-16 3.16 0.0089.4 741 4.3 175 4.1 IV-17 3.16 −0.064 13.8 964 11.8 130 4.2 IV-18 4.400.344 0.82 10 1.4 299 4.2 IV-19 4.40 0.204 1.54 11 1.6 290 4 IV-20 4.400.092 3.21 120 2.7 268 4.2 IV-21 6.23 0.508 0.77 5 1.5 300 4 IV-22 6.230.288 0.98 10 1.8 297 4.1 IV-23 6.23 0.120 2.01 44 2.1 284 4.1 IV-246.20 0.644 0.33 3 1.3 306 4 IV-25 6.20 0.304 0.94 9 1.4 298 3.9 IV-266.20 0.220 1.39 6 1.6 292 3.9 IV-27 0.50 0.145 1.98 31 2.4 284 4 IV-280.50 0.207 1.58 27 2.4 289 4.2 IV-29 0.50 0.135 3.53 46 2.5 264 4.1IV-30 0.50 0.160 1.82 33 2.4 286 4.2 IV-31 0.30 0.110 3.87 87 2.9 2584.2 IV-32 0.30 0.130 3.64 84 2.7 260 4.1 IV-33 0.30 0.186 1.82 74 2.5285 4 IV-34 0.30 0.221 1.47 62 2.7 291 4.1 IV-35 7.58 0.138 2.1 15 1.8283 3.9 IV-36 7.58 0.181 1.67 14 1.9 288 4 IV-37 6.23 0.608 0.45 4 1.2304 4.1 IV-38 73.50 1.832 0.12 1 0.8 308 4.1 IV-39 73.50 −0.304 13.1 98213.2 139 3.9

As shown in Table 7, it can be seen that the composite member obtainedby cooling the infiltrated plate in one direction from the side of theinfiltrated plate opposite to the side of supply of molten magnesium,here, the composite member obtained by cooling the infiltrated plate inone direction from the lower side in the vertical direction toward theupper side in the vertical direction, is of high grade, without defectsbeing present in a concentrated manner, in spite of being a large-sizedplate member from which a circular region having a diameter exceeding 50mm can be taken. In particular, all defects present in the compositemember for which cooling in one direction above was performed were sosmall that it is difficult to visually recognize them (not larger than0.1 mm) and a texture was uniform. In addition, it can be seen that thecomposite member obtained by performing cooling in one direction aboveincludes not only few internal defects but also few surface defects, andit is excellent in dimension accuracy. Moreover, it can be seen that thecomposite member obtained by performing cooling in one direction aboveis very low in coefficient of thermal expansion around 4 ppm/K, it isexcellent in adaptability to a semiconductor element or peripheralsthereof having a coefficient of thermal expansion around 4 ppm/K andalso in thermal conductivity, because there is also a sample havingthermal conductivity not lower than 180 W/K·m and in particular notlower than 300 W/K·m.

In contrast, it can be seen that the composite member for which coolingin one direction above was not performed is high in the area ratio ofthe defect portion above, and defects (pores) are present locally. Inaddition, defects present in this composite member were large enough tovisually be recognized. It is considered that this composite member waslow in dimension accuracy and also low in thermal conductioncharacteristics because of presence of such a large defect.

Therefore, the composite member obtained by performing cooling in onedirection above is expected to suitably be made use of as a constituentmaterial for a heat radiation member of the semiconductor element above.

Further, as shown in Table 7, it can be seen that the area ratio of thedefect portion can be made smaller by performing cooling such that atleast one of the temperature gradient at each temperature measurementpoint not less than 0.01° C./mm and the cooling rate at each temperaturemeasurement point not less than 0.5° C./min is satisfied. Furthermore,it can be seen that the area ratio of the defect portion tends to besmaller as the temperature gradient or the cooling rate is greater.Additionally, as shown in Table 7, it can be seen that the defectportions can be reduced as the temperature gradient is greater, in acase where the cooling rate is constant. Thus, it can be concluded that,by performing cooling in one direction above and increasing thetemperature gradient or the cooling rate, a composite member wheredefects are locally present is less likely.

Though a commercially available SiC sintered body was employed inEmbodiment IV-1 above, a composite member can be fabricated with the useof SiC powders. Specifically, a composite member including few defectsis obtained in spite of its large size as in Embodiment IV-1, forexample, by subjecting the SiC powders to oxidation treatment at 875°C.×2 hours, thereafter forming a powder molded body in the castutilizing tapping, slip casting, or the like, infiltrating the powdermolded body with molten pure magnesium as in Embodiment IV-1, andcontrolling a cooling condition during solidification as in EmbodimentIV-1. The powder molded body above may be sintered as appropriate.

Embodiment IV-2

A composite member including a substrate composed of a compositematerial made of a composite of pure magnesium and SiC and a metalcoating layer covering each of two opposing surfaces of the substratewas fabricated, and a state of defect and thermal characteristics of theobtained composite member were examined

An ingot of pure magnesium as in Embodiment IV-1 and an SiC sinteredbody were prepared as raw materials. In addition, the SiC sintered bodywas subjected to oxidation treatment as in Embodiment IV-1. Moreover, apair of plate-shaped spacers of 10 mm long×100 mm wide×0.5 mm thick andmade of carbon was prepared.

Here, cast 10 shown in FIG. 9 used in Embodiment IV-1, that is, a casthaving such a size as allowing arrangement of the spacer above betweenthe SiC sintered body and the cast, is made use of. Sintered body 20 andthe pair of spacers (not shown) are accommodated in cast 10 to which arelease agent has been applied as appropriate, and such a state thatsintered body 20 is sandwiched by the pair of spacers is set. Beingsandwiched between the spacers above, sintered body 20 is arranged inthe cast in a stable manner, and a gap corresponding to the thickness ofthe spacer (0.5 mm here) is provided between a surface of sintered body20 and sidewall portion 10 s of cast 10 and between a back surface ofsintered body 20 and lid portion 12 of cast 10. This cast 10 was loadedinto an atmosphere furnace as in Embodiment IV-1. Then, a composite ofsintered body 20 and molten pure magnesium was made under the conditionsthe same as in Embodiment IV-1, to thereby fabricate the infiltratedplate. In this embodiment, simultaneously with formation of theinfiltrated plate, molten pure magnesium flows into the gap between thesintered body and the cast provided by the spacer as described above, sothat a layer composed of pure magnesium is formed on each of the twoopposing surfaces of the infiltrated plate.

Here, the infiltrated plate above was cooled in one direction from thelower side thereof in the vertical direction toward the upper side inthe vertical direction so as to solidify pure magnesium, by making useof water-cooled copper or the like for achieving the temperaturegradient and the cooling rate as in sample No. IV-38 in Embodiment IV-1.Through the step above, the composite member (200 mm long×100 mm wide×6mm thick) was obtained.

The cross-section of the obtained composite member was observed with anoptical microscope (×50 magnification). Then, as shown in FIG. 10(II),it could be confirmed that the substrate made of a composite material inwhich the gap between SiC and SiC was infiltrated with pure magnesiumand the metal coating layer made of pure magnesium on the surface ofthis substrate were included. Composition of the constituent metal ofthis substrate and the metal coating layer was examined with the use ofan EDX apparatus, and the composition was identical (pure magnesium). Inaddition, it could be confirmed from the observed image of thecross-section above that each metal coating layer had a texturecontinuous to pure magnesium in the substrate above. Moreover, athickness of each metal coating layer was measured with the use of theobserved image of the cross-section above, and it was approximately 0.5mm (500 μm). It could thus be confirmed that this thickness wassubstantially the same as the thickness of the spacer above.

The SiC content in the portion where a composite of pure magnesium andSiC was made, that is, a portion except for the metal coating layer, inthe obtained composite member was measured, and it was 80 volume %. TheSiC content was measured as in Embodiment IV-1.

An area ratio (%) of the defect portion, difference in dimension (μm),and surface roughness Ra (μm) of the obtained composite member weremeasured. Then, the area ratio was 0.11%, the difference in dimensionwas 1 μm, and surface roughness Ra was 0.8 μm. It is noted that thedifference in dimension was measured, without excluding a thickness ofthe metal coating layer. In addition, regarding surface roughness Ra,the surface of the metal coating layer was measured.

Coefficient of thermal expansion α (ppm/K) and thermal conductivity κ(W/m·K) of the obtained composite member were measured as in EmbodimentIV-1, and coefficient of thermal expansion α was 5.1 ppm/K and thermalconductivity κ was 250 W/m·K.

From the foregoing, it can be seen that a composite member of high gradeand excellent also in surface property, without large defects beinglocally present, is obtained in spite of its large size from which acircular region having a diameter exceeding 50 mm can be taken and inspite of having a metal coating layer, by cooling the infiltrated platein one direction from the lower side thereof in the vertical directiontoward the upper side in the vertical direction as in Embodiment IV-1.In addition, this composite member is also expected to suitably be madeuse of as a constituent material for a heat radiation member of asemiconductor element, because it is excellent in adaptability to asemiconductor element or peripherals thereof having a coefficient ofthermal expansion around 4 ppm/K and also excellent in heat radiationcharacteristics. In particular, the composite member according toEmbodiment IV-2 can be plated with Ni through electroplating, because itincludes the metal coating layers on respective opposing surfaces of thesubstrate. Plating with Ni enhances solderability. Even in use in asemiconductor device for which application of solder is desired, thecomposite member can sufficiently adapt thereto. Further, since thesurface of the substrate itself is smooth as described above, thesurface of the metal coating layer is also smooth and hence plating canbe formed to a uniform thickness.

In the composite member according to Embodiment IV-2, a thickness or aregion to be formed of the metal coating layer can readily be varied byselecting a thickness or a shape of a plate-shaped spacer or the numberof used spacers as appropriate. For example, a composite memberincluding a metal coating layer only on one surface of the substrate isobtained by arranging a spacer only on one surface of the molded body(sintered body).

The present invention is not limited to the embodiments described aboveand it can be modified as appropriate without departing the gist of thepresent invention. For example, an SiC content in a composite member, asize, a shape, presence/absence of a network portion, a constituentmaterial for the network portion, composition of a metal component (forexample, composition of a magnesium alloy), a size of the compositemember, a thickness of a metal coating layer, composition of the metalcoating layer (a metal plate), a condition for making a composite, orthe like can be varied as appropriate.

INDUSTRIAL APPLICABILITY

The composite member according to the present invention can suitably bemade use of for a heat spreader of a semiconductor element, because ithas excellent adaptability in coefficient of thermal expansion to thesemiconductor element and peripherals thereof and has high thermalconductivity. The composite member manufacturing method according to thepresent invention can suitably be made use of for manufacturing thecomposite member above.

DESCRIPTION OF THE REFERENCE SIGNS

10 cast; 10 b bottom surface portion; 10 e end surface wall portion; 10s sidewall portion; 11 main body portion; 11 b bottom surface; 11 mmetal placement surface; 11 s SiC placement surface; 11 c couplingsurface; 12 lid portion; 12 i inner surface; 13 screw; 20 sintered body(molded body); 30 atmosphere furnace; 31 vessel; 32, 34, 35heat-insulating material; 33 heater; 40 water-cooled copper; 50 supportbase; 51 movable portion; M ingot; and S SiC aggregate.

1. A composite member made of a composite of magnesium or a magnesiumalloy and SiC, characterized in that said composite member has porositylower than 3%, said SiC is present in a form dispersed in said magnesiumor said magnesium alloy, and said composite member contains 50 volume %or more and 86.3 volume % or less of said SiC.
 2. The composite memberaccording to claim 1, characterized in that said composite member hasthermal conductivity not lower than 180 W/m·K.
 3. The composite memberaccording to claim 1, characterized in that said composite member has acoefficient of thermal expansion not lower than 4 ppm/K and not higherthan 10 ppm/K.
 4. A composite member, characterized by comprising: asubstrate composed of a composite material made of a composite ofmagnesium or a magnesium alloy and SiC and containing 50 volume % ormore SiC; and a metal coating layer covering at least one surface ofsaid substrate.
 5. The composite member according to claim 4,characterized in that a metal component of said composite material and ametal forming said metal coating layer have a continuous texture.
 6. Thecomposite member according to claim 4, characterized in that a metalcomponent of said composite material and a metal forming said metalcoating layer are different in composition.